I run the TKO500. Be sure to do a full 1,000 mile break in, then change the fluid. Make sure to use GM syncromesh. At first they are real notchy and I didn't try, but powershifting would be hard, but don't do that durring break in. It really sucked for me becuase I just finsihed the SpecIII break-in when my T-5 blew out. (spec install, t5 dies soon after, typical) So I dropped $2K for the TKO 500 and then right when I had it broken in, my motor started getting sluggish, the rings gave out, now its getting a new motor. Between break in and pulling the motor, I got some runs. It's so much easier to powershift, it seems to love it. Before my car outgrows this tranny, I'll have swapped to a auto by then.
The main difference between the 500 and 600 is the first gear and the input shaft options, otherwise identical. The only reason the 600 has a higher torque rating is because of the 26 spline input shaft, which is optional in the 500 and can be swapped over.