I wouldn't really say that PS3 is niched itself into any one or two categories. After all, PS3 has Little Big Planet and Spore, and stuff like Uncharted: Drake's Fortune.
It's just that the PS3 doesn't have enough of them, and enough of them being big hits. Going strictly on FPSs, the 360 has Bioshock, Orange Box (PS3 version almost doesn't count), Halo 3, Call of Duty, Gears of War 1/2, Left 4 Dead, etc. PS3 has maybe Resistance 1/2 and Haze, both with mediocre sales. (Haze is getting ported to 360/PC, actually, so it's no longer exclusive.)
Rosalina: But you didn't.
Robert: But I DON'T.
Rosalina: You sure that's right?
Robert: I was going to HAVE told you they'd come?
Rosalina: No.
Robert: The subjunctive?
Rosalina: That's not the subjunctive.
Robert: I don't think the syntax has been invented yet.
Rosalina: It would have had to have had been.
Robert: Had to have...had...been? That can't be right.