It depends on what you are going to use it for. The a600 is most compitable with classic ocs/ecs "a500" games, while the a1200 run the newer aga games but not so compitable with old ocs games. The a600 cannot run any of the newer aga games at all, but will run nearly all of the older a500 games.
the a600 is mostly just a smaller version of the a500+, with the exception of things like built in ide and pcmcia and different expansion slots.
But if you get a a1200, then i would highly recommend a hard drive upgrade if it lacks one, as you can use a hd installer to get alot of those troublesome ocs/ecs games working.