It really all comes down to the types of games you enjoy playing. The 360 has a year-long headstart on the PS3 so it has a larger catalog and more exclusives. The 360 has a lot of first-person shooter games like Halo and other shooters like Gears of War. Xbox Live is by far the most advanced online service of the 3 consoles so far, but the PS3 is catching up slowly but surely.
The 360 is cheaper, but a lot of the things that come included with the PS3 are extra on the 360 such as WiFi ($100 extra on 360), BluRay player, rechargeable batteries in the controllers, etc. So they end up being pretty close to the same price.
The PS3 has the BluRay player, which is a huge plus. You can get an HD DVD player add-on for the 360 for probably $50 now, but HD DVD is dead, so that won’t do you much good.
Again it really comes down to the games. Most games will come out on both consoles, but each still has a few exclusives that may guide your choice a little bit.