MS has been dropping the ball all year. They need to get back in the game. And get devs to release some more games. I guess there are more coming later in the year, but that's not really going to do them any good right now. Sony is still catching up, and that's never good. Especially with MGS4 coming out, and nothing to counter it on the 360.
I don't see how GTA4 can help or hurt either system. Of course, ideally more people would buy 360s to play it, but if they can't find one they won't. And most people won't know about/care about the exclusive DLC, so that won't affect their decision.
They need a lot more supply of Premium systems in retailers. In like the next two weeks. And they need a price drop at the same time. Not going to happen, of course. Sony is going to benefit from the launch of GTA4 by default.
Not to mention that their bribes and kickbacks to Warner Bros. have really been paying off for them because of the idiot fratboy crowd (who wants to watch BluRay movies on their 27 inch SD tv that's hooked up with the only cables included with the PS3 - composite.)