Hmmm. These are the factors I'd consider:
1) Killzone 3 and Bulletstorm have very similar review scores, as evidenced by thier metascores. The 'quality' of the games appears even and preference would then be a matter of opinion, not fact.
2) Killzone 3 is an established franchise with a fair amount of hype; Bulletstorm is a brand new IP, that sold, comparitively well.
So the op's question is valid; given the small initial sales of Bulletstorm on the PS3, and the launch of a competing established IP, would the game have been better served launched as a 360 console exclusive?
While I'm not claiming to know the answer I would guess, that launched as an exclusive, it would have sold similarly to the combined sales across platforms at launch. The question is, how will the legs on the PS3 affect the long-term sales.
