Flip a coin and get any.
But do read this before finalizing: *www.thinkdigit.com/forum/graphic-cards/140290-nvidia-drivers-opengl-issues.html
It is a serious issue imho if you are into rendering.
CUDA might be more "mature" as some people might say, but ATi Stream is better at video transcoding. Adobe applications like Premiere Pro have support for both CUDA and Stream.