Actually, I think it is up in the air which one of the two (sony MS) are going to bite the dust right now and it's probably going to take one more generation of consoles before we find out.
Sony has the Playstation brand going for them and a couple of decent IPs. They've lost alot of the exclusives they once had and the sales of their current system are sluggish for the moment. Their wildcard is the Blu-ray player. If Blu-ray wins the console war and they manage to put together a decent online offering, you can expect them to win over MS.
MS has X-box live, Halo, and Rare. Other than that they have no exclusives but they do have a larger install base and will be holding on to that lead for at least another year. If sony fails to overtake MS and MS gets more elclusives or takes more from Sony. Then (as much as I'd hate to see this happen) MS will keep on trucking and take Sony out.
All of this is MO of course and I am by no means qualified to make these assumptions. :)
