As others have mentioned, it really comes down to the gaming style and types of games.
360 is probably the preffered platform for online gaming. While there is fee for Xbox live gaming, the online system is miuch better maintained, and there are a ton more online players on the XBOX. Just compare the online activity for Call of Duty for XBOX vs PS3. The 360 also has some big exclusive series like Halo, Mass Effect, Left 4 Dead, and some others ... so 360 is the only system you'll current find them on.
The PS3 online is free, but doesn't have as much online activity. Where the PS3 shines is with its' exclusive titles. Games like Little Big Planet, the Uncharted series, Ratchet and Clank, Killzone, Infamous, God of War can only be found on the PS3.
I like both, but personally prefer my PS3. I'm not much of an online gamer, and for my gaming tastes, I've yet to see anything rival games like Uncharted 2 or Ratchet and Clank: A Crack in Time.