It's funny, MS delayed the Xbox so it could be significantly more powerful than the PS2 and gave a one year an advantage. Then it reverse on the PS3/Xbox 360. Now I think they are going to launch around the same time, but if the manufacturing rumors are true, then one may be delayed.
(There should be a game where you make Fanboys fight! )
I think people will be mostly loyal to their brands at first - then later it will dependent more on what games they want to play.
Oh, and my favorite Bieber video: (not for the squeemish!) http://www.youtube.com/watch?v=QxvrZwpW_-M