I just can't get an Xbox...no Show.
All jokes aside, that's what it comes down to for me - the games.
Most of my favorite games/franchises that I follow are on Playstation:
Uncharted/Last of Us, God of War, The Show, Infamous, Socom, Killzone - these are only on Playstation.
Xbox has some solid exclusives like Halo, Fable, Forza, and Gears of War, but of that list, the only one I care for is Gears and War but it's not enough to make me buy the console for it.
I had an original Xbox, but I found that it didn't offer me much of a difference from the PS2, so when the 360 came out, I was tempted to get it because it was out a whole year before PS3, but I decided to wait. And while XBL was definitely superior to PSN during the PS3/360 era, it was worth the wait because all my friends I played multiplayer games with had a PS3.
Whether it was COD/BF or playing in the DCC Madden/NCAA leagues, all that was done on PS3 so I've never really had any reason to get an Xbox.