For what its worth:
90% of the Tremec TKO's we sold have been the TKO 600 (.64). Most guys want fun gears (1,2,3,4) and when they hit 5th they want to cruise (low RPM). If you are road race guy with your car and need RPM's up when you hit 5th then you need the TKO 600 (.82). Also would need to look at TKO 600 (.82) if you have 3.08 or so rear gears and you can not drop your RPM that much. But if you have 3.36/3.55/3.73/etc rear gears.. should look hard at the TKO 600 (.64)
This calculator will help you look at RPM level:
http://www.classicchevy5speed.com/RPM_calculator.swf
The TKO 500 is not really that popular because of its VERY low 1st gear (3.27)
Hope it helps,