These two cards (PCI-E) are roughly the same price, and performance seems fairly similar.
Now, it is fairly probably that the system they will be going into will run Linux at sometime or another, so that is pushing me in the direction of nVidia, but I believe that ATi are improving in this respect.
If the performance is as similar as I believe it to be, then I'll probably be going for the nVidia part, but I just wanted to gather some opinions first.