XBox 360
Better games, wider choice of games, better versions of games (a lot of the 360 versions have features the PS3 counterparts do not), better graphics IMO, MUCH better for Internet gaming, Sony still haven't got that right.
Developers also seem to prefer programming for the 360, much easier, which means in the unlikely event of a game getting delayed it is more likely going to happen to the PS3 version.
Also you get a lot more HD space.
Oh, and it's backwards compatible with cheap Xbox games (which are still rather good).
Now I'm no fan of M$, but as a 360 owner myself and having used a PS3 and looked at games on both systems, these are the facts I have discovered. Not to mention Sony treat their European customers like s***.