It really comes down to 2 things:

1. Which titles exclusive to one platform or the other do you want to play?
2. Do you want it to be a Blu-ray player?

Both have their advantages and disadvantages. The Blu-ray thing isn't *only* beneficial for the ability to play movies. It also lets them put big games, like FFXIII, on one disc, as opposed to multiple DVDs on the 360. If you don't mind swapping discs, no big deal, but it's a point to be aware of, especially with large RPGs.

I personally went with the PS3 because I'm a Gran Turismo addict, and that's not going to be available for 360. Between that and the appeal of the blu-ray player, I was sold on the PS3.