I've designed boards with both ATi and Nvidia chips and the drivers are largely a case of finding a good one. Nvidia tend to have a lot more beta versions around, which can be a good thing, if you have the time to play with them.
I've mostly had Nvidia's at home, but the X1950 Pro is an excellent card and I'd highly recommend it.
Intel owns the 3D market with its chipset graphics (around 40% market share). Of the discreet market players ATi and Nvidia tend to be neck-and-neck in the 20-30 percent market share, depending on who's launched what lately. ATi used to be a little bit ahead, as the 7xxx Nvidia chips weren't as good as their offerings, with the 8xxx Nvidia seem to be on top and gaining share. The last figures I've seen have Intel on 38%, Nvidia on 33% (the highest share they've had for many many years) and ATi on 20%.
Handy graph:
http://www.xbitlabs.com/images/news/...007_trends.png
I've only seen MA issues on an old Nvidia card, but really haven't done enough testing for it to be relevant.
Handy-Dandy (you yanks are rubbing off on me) video card comparator:
http://www.gpureview.com/show_cards....=511&card2=550
Performance charts:
http://www23.tomshardware.com/graphi...=706&chart=313
Although there's a lot of cheating (in drivers) for 3DMark...
John