I'm going to lean toward MS on this one, but just barely.
I think it's a matter of opinion. I really love Fable II and Viva Pinata. Like I wanna marry them.
And I've been married to Halo since day one.
While I like a lot of the Sony stuff, especially LBP and Uncharted, I just don't have the same attachment to it. The Sony exclusives that I was into were Devil May Cry, Final Fantasy, God of War and GTA, and three of the four obviously jumped ship.
So it's a taste thing. If I liked Metal Gear or Gran Turismo more, I might change my mind, but both of those are awesome franchises that I just can't get into.