When I had my '69 Camaro, I had bought an original TKO trans (which is not a TKO 500 version). By the time I got to the point of installing it the 500/600 series came out. I sold off my TKO and bought a TKO 600. The rear end had a stock 10 bolt with 2.73 gears. Way wrong gears but I had a 12 bolt I was going to put in with 3.55's but sold the car before I ever got to put that in.
As it was, with the 2.73's (and ZZ4 engine) it got off the line fairly well. Not quite as hard as a late model SS Camaro that I raced but once moving I hung right on his door until we backed off at 100 mph. With this gearing, I could do 120 mph in 3rd gear! Surprised more than a few passengers.
My take is this, I am more of a fan of a close ratio box and to me, I felt there was too big of a space between 1st and 2nd on the 500 model. Now keep in mind I was setting this car up to play on twisty roads/autocross so the tigher spacing of the 600 kept it more in the power band during shifting. For just a cruiser, the 500 gearing will be a little easier to start off the line but depending on your engine, you could lug it a little when you shift from 1st to 2nd.
In the end, either one will work and it comes down to your needs/wants. On MAJOR critical thing on either one is dialing in the bell housing, to get the required specs for centering. You have to measure this with a dial indicator and just don't bolt the bell housing on and then the trans. Almost ever issue I have hear with the Tremecs has come down to not setting up this runout correctly. I was very methodical with doing this and my worked perfect and shifted easy. I LOVED that trans and if I had the funds, I would be putting in one in my '65 instead of the AX15 setup I am going with.
Good luck!