If you have a PS2 and want to play games on it, do not even consider the 40GB model. One of the coolest features of the other PS3s is the ability to upscale your PS2 games. It works great.
With that said, I prefer my 360 by leaps and bounds over my PS3. That may change next year when the PS3 starts rolling out their AAA lineup but even now, you have two years of back catalog to play on the 360 (at least 10 games worth playing, probably more) while the PS3 has R:FoM and Heavenly Sword. You can rent a PS3 and beat them both in a weekend. There are other games out there, just not much that has impressed me thus far.
Take a look at the games and then decide what is best for you.

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