I prefer Sony due to a combination of great hardware, plenty of new ips and generally being more pro consumer IMHO. The third party support along with ever improving online features is a big plus.
On the other hand I'm not a blind fanboy and I will only support them as long as they continue to cater to my tastes.
Microsoft I like in the PC space but have not been a fan of their consoles since I owned the original Xbox. I find them to be a bit too restrictive. Examples include not allowing non proprietary harddrives as well as restrictions on many accessories. Halo is great along with third party support but they are a notch below Sony in my book.
Nintendo never did it for me. Even before Sony and Microsoft entered the console space I preferred Sega over Nintendo. Not enough mature games and the hardware is not up to par power wise. Not to mention horrible 3rd party support and far inferior network features compared to Microsoft/Sony.
