New article in Chevy High Performance compares 5 and 6 speed trannies but the T5 isn't on the list, just the TKO. They say the main difference is whether you want to run a modern electronically fuel injected LS1 and turn low rpm on the highway to get good mileage or a carbureted motor that cruises at a higher rpm--the carbureted motor will bog in 6th unless you go 4.10 in the rear or deeper. They also say that the 5 speed shifts better than the 6, doesn't need an electronic reverse cutout and weighs 30 lbs. less.
As long as I run a carb and monkey with the same old stuff I'm used to (the reason I don't like 4th gen--can't get to the top of the motor) I'll stick with a built 5 speed and 3.73's. Some day I might go with the LS1 and T56, but installed as a unit--when I get tired of playing with jets and such.