Well if it got unlocked then you must have a good reason for this post.
That said i'll chime in for drivers.
I've owned alot of video cards over the years, and used alot of different OS's. Currently i spend 70% of my computer time running linux, only boot windows to game. Years ago was the opposite, spent all my time in windows and only used linux to play around with sometimes.
Nvidia is leaps and bounds above ATI in drivers for linux, and they also offer drivers for BSD while ati does not. When i was running a 4830 sometimes the ati linux drivers would be so broken that the desktop effects would be choppy, its kinda sad that a card with as much horsepower as a 4830 cant handle desktop effects. the 4890 was better, desktop effects never lagged but 3d performance was all over the map from driver update to driver update.
Then there was my file server, i built a AMD AM2+ system for a file/media server with a 780G mobo, used the integrated ATI 3200 graphics running ubuntu. It crashed every 2 days give or take 12 hours. I swapped out every piece of hardware one piece at a time(mobo, CPU, RAM, PSU, even the case) in that system with my girlfriends computer(also a AM2+ platform, 780G mobo) and it still crashed every 2 days. While gf's comp was still stable as a rock(she was using a 4830). Finally decided it might have something to do with the integrated video drivers and bought a passive cooled Nvidia 8400GS, the system has now been running for 145days 12hours and 13min without a reboot so i guess its safe to say it had something to do with the ATI linux drivers for the 3200 graphics.
From a strictly windows standpoint the drivers are pretty much the same across the ATI/Nvida camps, sure ATI has CF scaling issues and every driver release breaks as many CF setups as it does fix ones that weren't working, so if you are going multiple GPU it just makes sense to go Nvidia. Most users however do not go CF/SLI, only a small number. And Nvida has had buggy drivers in the past, especially back in the TNT/first Geforce days and if i remember correctly they even released drivers that bricked some cards, something about turning the fan off and them overheating. So both sides have had issues.
Bottom line if you are using a single card only and only use windows it doesn't make a difference which side you go with. If you run multiple GPU's and/or also use linux then Nvidia would be a better option IMO.