I too have both consoles, and a Wii, however if I could only have one I would go for the PS3. I haven't seen a great deal of difference in most games, but I do have FIFA 08 on both and prefer the PS3 version.
Other than the quality/range of games you should also consider what else you get with the PS3. The motion sensor pads are great, especially with games like Ratchet & Clank. You also get the wifi built in, controllers with a built in play & charge kit, blu ray drive and free online gaming, all this would cost a small fortune to get for the XBox 360, XBox live itself is £40 per year.
As mentioned before the acheivment points are good on the Xbox, however in most games on the PS3 there are similar acheivment points to be unlocked, they are just based within the game and not open for everyone else to view. I played the Darkness through 3 times to unlock all of them.
One more positive for the PS3 is that as you said you have a PS2 you will be used to the controller, as it has barely changed.
The XBox 360 is a great machine as well though, I use them both about the same amount for games ( I use the PS3 more overall but that's because of the Blu Ray player ) and whatever one you choose I'm sure you'll be happy with it.