I have nvidias as well as atis here and for my newest pc i went for nvidia.
Because fglrx is still a pita.
On one of my machines i get a lock whenever switching from xorg to console (Radeon 9600 i think?), 'BigDesktop' (for one of my notebooks is) not usable either (Radeon xN00), at least theres no way to get a setup working that enables it when plugging my laptop onto my dock).
Nvidia on the other hand works great, no troubles at all (some older models, newest being a mobile nv?160, not sure about the naming).
From personal experience on quite a bunch of boxes i'd never recommend ati+fglrx, it hurts (because i like ati better :-/) but their driver just sucks (esp when coming to multi-head/xinerama/... setups).
Maybe in a few years their commitment to being more open will have paid out and we'll be able to get decent support for our ati cards when running linux, until then nvidia :-/
oh and btw you already made your decision :
Nvidia ...I DO want it to have roughly the same performance vs a Windows version of driver.