I heard one wag describe Vista as "XP, Millennium Edition".
It would be an interesting dynamic. I know Vista is supposed to be more compatible with the new upcoming Phoenix Core Managed Environment (cME) standards-based grid computing system. Microsoft has been working closely with many other companies to develop the standards for the grid system for years now. This gives Vista an automatic leg-up from certain governments and security information brokers. This along with the rumors that Microsoft is poised to launch into the world of communications satellites makes me think there is more to Vista than meets the eye, but that could just be a latent predisposition to conspiracy paranoia cropping up.
On the other hand, Microsoft seems to be legally constrained from bundling software with Vista the way they have in the past with all other OS's.
It's hard to say, but my best guess is Gates would have gone with XP if he was presented with this scenario as his only options a few years ago. Today, I think he would run with Vista.
As far as market share goes, I think an XP upgrade would have won out.