When it comes to choosing a 4, 5 or 6 speed transmission for your car you have to take several things into consideration.
1) How and where is the car going to be driven?
2) What kind of gears do you have in the rear end?
3) Do you want it to shift fast, such as speed shifting?
4) How often will you be driving above 100 MPH?
There are more factors involved but these are the main concerns when choosing the right manual transmission for your car, whether it's a street car, a weekend drag racer, or a road racer.
A quick look at the advantages and disadvantages of different transmissions is that more gears distribute the engine's load over a narrower RPM range. In other words, more gears = keeping your engine in it's "happy zone" better, much like riding a 10 speed bike is a lot easier over varying terrain than a single speed Schwinn or something is.
One thing to remember is that pretty much all 4 speeds end-up with a 1:1 high gear (as do all 3 speeds and 3 speed automatics), so thinking about rear-end gearing is important for higher speed driving, such as down the freeway or very high top-end speeds. If you want a car that will leap from 0 - 60 MPH quickly, you need low rear-end gears. The problem with low gears is that you end up running down the freeway at a high RPM and always feel like you need another gear to shift into, so you are either stuck with a car that you can't drive as fast on the freeway because your rear end gears are too low, or you have to run taller rear gears which kills your get-up and go from 0 - 60 MPH. This is where overdrive transmissions come-in, but we'll get to those later.
The advantage of a typical 4 speed, such as a Muncie M20 or M21, Borg Warner Super T-10 (not to be confused with a standard "T-10" which is MUCH weaker), a Richmond 4 speed (which IS the Super T-10 being that they bought the rights to build them from Borg Warner), a Jericho, etc, kind of transmission, is that they shift really well and really fast with their external shift linkages and are really strong. I didn't mention teh Muncie M-22 "Rock Crusher" for a reason... because those are SO rare, it isn;t even funny. 95% of the guys who claim they have one end-up just having a typical M-21 instead. They only came in a VERY limited amount of muscle cars, and even with those, most M-22's were still an "option" that you had to order. For instance; my 69 427 Tri Power Vette had an M-21 in it, not an M-22, so contrary to what a LOT of guys believe they have, most are just M-20's and M-21's. Now, stick-on something like a Hurst Super Shifter or a Vertical Gate to any of these and your shifts are literally lightning fast between gears, plus the fact that these transmission actually FIT into pretty much every vehicle out here with no floor modifications, but again, you're limited to a 1:1 4th gear, which limits you either for top-end speed or 0 - 60 jack rabbit starts. You kind of have to make a decision for which one you want because you can't have both. Most guys split the difference by going with a little lower rear gear so they get decent 0-60 launches, but still don't tach-out too much on the freeway. 3.73's are usually the "go to" gear for this situation. With an average size tire you can usually run down the freeway at 65 MPH and still be under 3,000 RPM with 3.73's and a 4 speed. For guys like myself who used to run 4.88's on the street with a Super T-10, I had to cruise at about 45 MPH on the freeway which would still be at about 3,000 RPM or so. At 65 MPH I would be in the 4,000 - 4,500 zone, but for a 1970's 10 second street car, that's what you had to run if you wanted to be quick on the street... and I was. Not everyone is willing to go that far though, and I don't blame them.
There were wide ratio and close ratio models of almost every type available, which meant the wide ratio had a lower 1st gear for better take-offs from a dead stop and more RPM between each shift, and teh close ratio had a taller 1st gear with less RPM between shifts. These were mainly made for engines that didn't make a lot of torque and needed to keep the RPM range within a given area so the engine stayed in its power band better without dipping too low into the RPMs to bog it down. Either way each of those still had the 1:1 4th gear. So, with a 4 speed you are pretty much forced to make a choice between being "quick", being "fast", or being "mediocre" by having to split the difference between the two.
Now we have 5 and 6 speeds. Let's start with the 5 speeds. First and foremost! Do NOT waste your time on an old T-5 5 speed!! Those are weak and they will not hold up to anything making any kind of power, period. They get sloppy REAL fast and sooner or later they will let-go... and it's usually sooner, unless of course you don't have much power and then it doesn't really matter. The cool thing with 5 speeds, such as the TKO 500 and TKO 600, which are both VERY good 5 speeds, is that you have numerous choices between gear ratios, including 1st gear and overdrive. You can get them with really low 1st gear, such as a 3.00 ratio to get you off the line better when you have taller rear-end gears, and you can get them with taller 1st gear, such as a 2.56 ratio for when running a low enough rear gear. The overdrive is also variable in that you can have a "deep" or "shallow" one. A shallow overdrive is usually something like a .82 and a deep is usually in the .64 area. A shallow OD is usually used in something like a road race car where 5th gear is being used more like a "gear" rather than an overdrive. This is handy for the back stretch at high speeds. The deep OD is used for just that, an overdrive, for cruising down the freeway at a low RPM for better mileage and less wear and tear on the engine over long periods of driving. This is real handy for cars with low rear-end gears so they can have really quick jack rabbit 0-60 MPH acceleration, yet still be able to cruise down the freeway at a decent RPM. On the flip side, you don't want to run too low of a rear-end gear because 1st gear becomes useless. In other words, 1st gear ends-up being more like a "granny low" gear than a useable 1st gear, and that's not good to have either, unless you drive up steep hills a lot and can use a really low take-off gear like that.
The downside to pretty much all of these nice 5 speeds is two fold. 1) They are big and don't fit into most tranny tunnels without having to do a little dimpling, or modifying. Sure, we put TKO's in LOTS of 65 & 66 Mustangs and such, and they fit with a little finagling, but that's the key... a little finagling. Don't just "expect" one to fit without the probability of having to do some sort of tranny tunnel fitting. And 2) they tend to not shift very fast with their rail type shift linkage.
Trust me, in my younger days I was a lightning fast shifter, and I'm still really, really fast at speed shifting, but not with any of these 5 speeds. For some reason, no matter how fast or strong you think you are, the shift linkage always wants to stop and pause in neutral before going into the next gear. In other words, a 4 speed with a fast Hurst shifter on it can simply jump from 1st to 2nd in one short, smooth motion, but not a rail shifter tranny. They always seem to go "click ... click" from one gear to the next with that pause in neutral. It's never as fast or as smooth as the old 4 speeds were. I'm sure some guys will be saying they can shift theirs really fast. Well goody two shoes, they've evidently never shifted something like a Super T-10 or an M-21 with a short throw Hurst shifter on it because hands-down, those definitely shift quicker and smoother.
Next comes the 6 speed choice, such as the common T-56, and for 95% of the guys on the street with "hot rods" or muscle cars, this tranny is a real no-no. 6th gear is simply way too tall for use in normal driving conditions (in most cases). Here's why; what kinds of vehicles do these 6 speeds come-in? They come in things like new Vettes, Vipers, etc that are 180 - 200+ MPH cars. At what speed do you think they need to be going to shift into 6th gear and take it up to that car's max speed? And just how much speed and RPM do you think are built INTO one gear? In other words, how fast can you typically go in 1st gear? How about 2nd or 3rd? each gear has a given amount of RPM built into it, no matter which gear it is. The rest is all up to your engine's power, your rear-end gearing and youyr terminal top speed. In other words, in cars that these types of trannys come-in, they don't usually use 6th gear until they are already going over about 80 MPH (or more in most cases) and that have enough engine torque to pull that gear.
Let me give you a scenario. A friend of mine built a hot little 66 Chevelle with a decent small block in it. It was probably an honest to goodness 460 or so HP and at least that much in torque. With his 3.73 rear gears, he couldn't shift into, or use, 6th gear until he was going about 100 MPH because the revs were too low for his engine to pull that tall of a gear. If you have a car that tops-out at 200 MPH (like a ZR-1 Vette or a Viper) do you think you shift into 6th gear at say, 65 MPH and carry that gear another 135 MPH? Can you go 135 MPH in ANY gear? Hell no! So to use a gear like that, and to go 200 MPH, you'd be shifting into 6th at probably about 140 MPH or so with the engine revs up enough to be well into its max power band area to be able to "pull" that tall gear. Just think about how well you can accelerate a 10 speed bike when going 10 MPH in 10th gear. Good luck with that! 10th gear on a 10 speed is for cruising down the road at about 30 - 35 MPH, and you have to be IN motion and literally stand up on the pedals when you want to go faster because that gear is so high. A car's gearing is no different. if you are in 6th gear cruising at 70 MPH at 1,500 RPM in any kind of performance engine, you'll be hard pressed to accelerate decently without down shifting 2 gears (at least) to get the revs up into the engine's power band to get-up and go. In fact, in some cases, a lot of "hot" engines with big lumpy cams, huge intake manifolds, big carburetors, etc, the engine won't even be able to maintain a cruising speed at that low of an RPM because it will fall on its face because it is so far under the RPM range where it makes its power. I've seen cars like that have to down shift every few miles to get the car back up to speed again because it was lugging so badly at such a low RPM. Believe it or not, when you are lugging like that, you actually use more gas than you would if the engine wasn't trying to work so hard, so the "theory" of getting better gas mileage with an overdrive tranny goes right out the window if the overdrive is too tall and causes the engine to lug down too much.
My point (in short) is; in 99% of the cases for most guys out there with most types of muscle cars or hot rods, 6th gear is almost useless for normal driving. Another bad point is; the T-56 is a HUGE tranny and usually requires quite a bit of trans tunnel modification to get one to fit. You're probably better off going with a 5 speed, and for around town or lightning quick shifts, a good 4 speed might be your better choice.