It depends on what kinds of exclusives you want. Xbox has some good ones (Halo), Playstation has some good ones (...Little Big Planet? Honestly, I hate most PS exclusives except that and Heavy Rain, which are both really overpriced).
Oh, but Playstation does have Demon's Souls. Both systems have the sequel, Dark Souls, but every single person should play Demon's Souls just once, because it's easily in my top 3 favorite games of all time. That said, I don't even have a Playstation 3, yet I own a copy of Demon's Souls. That's how often I play it whenever I go to a friend's house. It's that good.
Regarding the hardware, they're both pretty similar, but each has a couple of things that top the other. The 360 caches its memory as one block of 512mb, while the PS3 caches it as 2 256mb ones. This means that Skyrim on the PS3 runs out of memory far quicker than the 360, yielding tons and tons of lag once you get far enough in the game. That said, the 360 version will eventually face the same problems, it will just take longer to reach that point.
PS3 discs are blu-ray and can store a lot more on the disc than the 360 discs.
The various PS3s are a possible point of contention, too. Some of them support backwards-compatibility, some don't at all. The originals allow you to install Linux onto it, then Sony removed that feature entirely (last I checked), and even removed this feature from the OLD PS3s via update. These two could be possibly frustrating.
I, personally, can't stand the Playstation controller. At all. It's the worst controller for holding in your hand since...shit, it's even more uncomfortable to hold than an N64 controller, and that's saying something. However, they now have some Playstation controllers SHAPED like 360 controllers, removing that issue for those with the cash.
Regarding live/such, updates for the 360 are way, wayyy faster and less likely to poop out midway through compared to the PS3, the PS3's is obviously free, but the 360 has a larger community and rarely has any downtime (any that is is because of the studio, not Live).
I'd say get what your friends have. I'm still annoyed at a few friends for buying the PS3 Dark Souls, when I have the 360 one. Even though we found out later that we wouldn't really be able to play together, because the server you join is randomized at start-up, since we all have 360s but only half of us have PS3s...
Play whatever lets you play with the most people that you know. But don't ever play Gears. I'm a 360 gamer, but that game is boring as hell and sets gamer stereotypes back 10 years easily. I find Killzone/Resistance/Uncharted/God Of War to be just as boring, and other than that, you just have niche titles as exclusives, like Heavy Rain or Demon's Souls.