Both have positives and negatives but at the end of the day it comes down to the games. Look at the list of games (there's probably one on wikipedia) for each platform and buy the one with the more games that you like the sound of, when in doubt, load up youtube & watch a level or 2.
Based on several years of use (and only really talking about platform exclusives), I'd say that the PS3 has more special games and the 360 has more good games.
There are several games on the PS3 where I sit back and say "Wow, now THAT is why I bought this thing!" but thats balanced by the fact that I have 9 PS3 games and 50+ Xbox 360 ones. On a cost to benefit ratio basis the 360 is a clear winner but playing stuff like the amazingly engaging Heavy Rain or Gran Turismo 5 with a proper racing wheel (its pretty crap with the dualshock controller) makes me think that the PS3 was a worthwhile investment after all