DX10 is DirectX 10. It is the next generation of shader modules for games and such. It is ONLY available in Vista. It will not be available for XP at all.
DX9 Scene
DX10 Scene
Notice the difference.
My Vote goes to Vista. Since i have used it since Beta 1 back in July of 2005 i have seen it come a far way. Not that it is final it does have a couple of bugs but overall i think it is a step ahead of XP. Plus i have no problem running anything on it.