The answer is: it depends. The Revell T600 represents a 1986-1989 truck while the AMT/Ertl T600 is a 1990-1994 body with a 1970s chassis, engine, & suspension. I have never built either kit, so I cannot comment on the quality; though I assume the the Revell kit is better.
There are some good articles here on correcting the AMT T600:
http://public.fotki.com/modeltrucks25thsca...s/car_modeller/
If you can find/afford a Revell T600, get it!