As said before.
Games - Multiplatform titles are always there. If your into shooters you still get the exclusives like Gears of War, Halo, and Call of Duty timed DLC. MS is still throwing the 360 a bone this year by released Fable Anniversary HD this fall/winter. You can't discount all the games released the last 8 years either.
My friends have it - People buy 360's to play online with their friends. If all their friends have 360's then they will most likely buy one to.
Cheaper - The XBox has always been cheaper then the PS3.
It's just that simple.