the only reason to buy one system over another between the PS3 and the XBox 360 are exclusive games.
look at the games list, and find the games than are only on the 360 or only on the PS3. decide which system has more of the games you like, and buy that system.
otherwise, for the most part, they are the same. PS3 has a few more hardware features(BlueTooth, built-in wireless, the ability to swap your hard drive out with any laptop HD), 360 has more software features(more robust online service, Netflix, etc.).
personally, I like my PSP, cause I can play my games anywhere, rather than stuck on a couch in front of a TV. and the PSP is perfectly capable of Console quality(say, PS2-ish) full games.
I will say though, that the 360 has the greatest controller ever.
but if I get anything beyond my PSP, it will be a PS3. for the simple fact that, I can buy a PS3, install Linux on it, and then I won't need to buy a new computer.