Well I have a 360 so I already have bias there. I of course would have bought Microsoft's X-Box 360 if Nintendo did not produce a console this generation. Nintendo and Sega have always been my favorite hardware manufacturers and if Nintendo disapeared I would have to find new hardware to game on.
I'm not about to stop gaming over the loss of Nintendo though it would be a major setback and it would cut my gaming time substantually.
I would go 360 mainly because of Microsoft's current Nintendo-Esque development style. You can't possibly deny that Microsoft isn't taking alot of Nintendo's plays from their past playbooks and playing them on the feild. Targeting the hardcore demographic that has always been Nintendo's strong point. Shooters where Nintendo used to dominate (N64) even party games and the aquisition of Rare.
Microsoft at this point is Nintendo 2.0, they can't possibly live up to the standards of the real Nintendo but they are trying as hard as they can to become the new Nintendo. Putting more weight on internal development of first party games over third party is another major thing Microsoft is acceling at.
So in the end I love Nintendo more then any other company but if I didn't have a Nintendo platform to choose from I would go with Microsoft!