DX10 is better, play crysis in both DX modes. You will want to continue using DX10
You guys can flame vista all day long, its a great OS. And one of these days you (yourself) will probably find yourself upgrading. Just like windows 2000 users finally found theirselfs going to XP. Im sure the same thing was being argued then about the all wonderful windows XP.
When your using XP for years, and you do a major change such as swapping to Vista, your not going to like it because your so used to XP's configuration. I found myself a little dissapointed in vista the first week of using it, but one i learn my way around. I would never want to use XP again, vista has so much more to offer for me.
Its like the first day of school, no one likes it.