I don't think any of those two methodologies are good enough at all. Multiplatforms, even when they are all rated AAA, can be of a very different quality ( PC Mass Effect and Dragon Age Origins were very superior to other versions of the game, not only visually but mainly gameplay wise). And if you only consider exclusives you might find yourself picking a console with lots of exclusives (Wii) but little multiplatform support.
I guess a mix between those two would be better.







