If I remember it right, there are fewer exclusives on the PS3, so the easiest way to settle it is to look at PS3's exclusives list, and make up your mind about whether you want them or not.
Other things you could look at are:
- quality of multiplatform titles (usually the 360 has the advantage in terms of ports; early PS3 ports were especially poor)
- do you want to play online, and if so do you mind paying the yearly Xbox Live fee for it?
- are you interested in Blu-Ray movies, or is DVD enough for you?
- which controller do you prefer?
Frankly, the PS3 and 360 aren't that different. One could even say Microsoft and Sony should have merged their consoles (which wouldn't work for obvious reasons, but you get my point). Therefore, to make a decision you have to look at those small differences along with the exclusives which are the most important difference.
My Mario Kart Wii friend code: 2707-1866-0957
