Honestly I'm not a big fan of anti-aliasing, I tested it often in the past and I have a hard time noticing the improvement in quality not to mention that it lowers your FPS. I try to stick with 4X MSAA and not bother too much with it. But in most games I would remove AA or put it back and would have a hard time noticing the difference.
Anyways, you should get the 7850 there's no contest. I don't think you can say that nvidia drivers are better anymore. Not a lot of games have PhysX, for instance my last 2 video cards were nvidia and I never ever used PhysX. In other words, I don't see any advantage nvidia vs. AMD, or maybe there are some but with things that I'm not using anyways so I don't bother I just play games. I don't have any preference, I'm looking to replace my video card and I might change for AMD, I've not decided yet, but in my book AMD and nvidia are pretty close to each other for gaming performance.