I'd go for the A1200, as it's significantly better in terms of specs. It can play most games straight off, including the AGA ones, which are completely out of reach of the A600. There might be a few games which won't work straight away, but usually you can get them to work with a little tweaking. Ultimately, if all else fails, there is software like "Relokick" (think that was the name), which effectively turns an A1200 into an A500 (temporarily, of course ;-)).
Oh, and should you ever want to upgrade, the A1200 has more possibilities.
Kay