Both cards are not for gaming at all. If you want to use vdpau for xbmc or so go for nvidia. Also you get more often driver updates with support for new kernels and xservers. If you intend to play get something faster, maybe gt640+.
Ah, should have known! I always forget that! About the support for new kernels and xservers. It's worth going Nvidia just for that. Yep, it will be gt 640 or if I can budget $100, I'll get the gtx 650!