[flylwsi]

this isn't an easy question...
in a car that has both available, like a miata, the final drive isn't much different...
it's just that they push the rest of the gears closer together...
it's all about the gearing though. you can have a great 5 speed, and not need the 6th gear...
the 6th gear in most cars on the street, like the mustang/viper/vette/f-body cars is an overdrive only, and used to keep your freeway mileage respectable...
in other cars, such as the miata, the gearing of the "overdrive" in the 5 and 6 speed is the same, but the rest are closer...
2 different schools of thought...
1. you add an overdrive to a 5 speed (i know that's oversimplified, but that's kind what it is with the tremec in those cars)
2. you keep the top gear the same, and push the rest closer... (again, oversimplified, but for all intensive purposes)

this is why you can't compare my 5 speed prelude to a 6 speed supra, and say that the supra is better/worse b/c of the # of gears...