Well, how attatched are you to the 2.73 gears and what are your long-term engine plans? If you're planning on staying with the 350/330 it'll have good low end torque and a decent powerband, so the RPM drop from 1-2 shouldn't hurt you much, but the low RPM at cruising speed may be a bit odd. With a 26" tall tire, the TKO500 will put you around 1750 RPM at 70mph and the TKO600 will put you at around 1650 RPM. That's pretty low. If you downshift into 4th at that speed, you end up at ~2550 RPM on either. 3rd puts you at 3450 and 3300 respectively. To me, that means you're gonna be going from 5th to 3rd on either trans if you really want to accel on the highway. Probably an OK fuel economy tradeoff if you're going to do a lot of highway driving.
FWIW, I currently have 2.73's and just moved from a Super T10 with a 3.42 1st gear (9.3 overall 1st gear ratio) to the TKO600. I kept playing with the numbers and basically concluded that I was willing to lose a bit of accelleration in 1st gear (for now) with the TKO600 in exchange for a better ratio spacing. I figured the TKO500 would have been good for now with a better launch (8.9 overall 1st gear versus 7.8 for the TKO600), but 5th would be too low with the 2.73 rear (even with Michigan 70+ highway speeds) and I'd want to change them out anyway. So I figured if I'm gonna end up doing rear gears eventually anyway, the TKO600 has better spacing. Still haven't got everything together yet, so I can't comment on how it feels with my lowly 307 H.O. and 2.73 rear.
Eventually, I'll have 3.42 rear gears, which will put me at 2050 RPM at 70mph (still pretty low) and gives me a nice 9.8 overall ratio in 1st gear. Wth those gears, 4th gear at 70 gets you to 3200 RPM, which is in the meat of my powerband for passing. Until then, I'll have an internally shifted 4-speed with an "extra" gear that I rarely use.
Other (possible) concerns with the TKO500:
-- bigger RPM drop between 1st and 2nd (if you shift at 6k, the 500 puts you at ~3600 RPM in 2nd and the 600 puts you at 3900. Depending on the powerband of you motor, this may or may not matter)
-- wider gear spacing will tend to make it harder to shift "fast" on the 1-2 shift with the TKO500 (need to change input shaft speed by a greater amount; syncros need to work harder. Not sure if this would be enough to notice or not.)
-- higher torque capacity on the TKO600 (I doubt I'll ever end up with more than 500 ft-lbs, so not an issue for me)
Personally, based on your intended use (backroad cruizing, off-the-line performance, not much 70+ highway), I'd consider changing rear gears at some point regardless of which trans you end up with, otherwise you'll have a 5th gear you will probably rarely use. That said, either trans should be fine launching for now with the 2.73's (wouldn't worry about burning the clutch as long as you don't start towing trailers). I think the TKO600 with a 3.42/3.55/3.73 rear is the best overall compromise for you. If you don't want to drop the $ on the rear just yet, the TKO600 with the 2.73's will do you fine for now.
