Both consoles have their ups and downs when it comes to specs, capabilities, and game exclusivity... but one thing separates them apart completely, and that's the online community.
XBOX: Premium community, $60 a year being the cheapest option. PS3 is absolutely free. The downside to the PS3's community being free is that it leaves room for little kids, people with shitty connections, and otherwise people who don't really need to be playing online to be playing online. On the 360 you still get a number of kids who are spoiled by their parents, but I used to think it was bad until I played a game of CODMW3 on my roommate's PS3. It was HORRIBLE. Nobody could host a game, 5/12 of the players sounded prepubescent with maybe 3 having a voice sounding like a dog whistle, and the quality of the gameplay was terrible... I felt like a God. And I suck at COD. These kids were just bad, and the connection felt like it was being run on a server from a garage sale. PS3 was just a traumatic experience in general.
Xbox > PS3 for online play. Otherwise, it's about dead even for me. I don't even play video games that often, but fuck the PS3's online community. (Most of it, at least)