I would definitely take a 5850 over a GTX285. For the reasons below:
1)Cost - The 5850 is less expensive
2)Performance - The 5850 outperforms the GTX285 in virtually every case
3)DX11 - 285 is only DX10 and the newer DX11 games are coming this year, why would you deprive yourself from eye candy?
4)Overclocking! - The 5850 can be overclocked to 5870 speed and beyond, check out this article. From what it sounds like the 5850 can go to 5870 speed without any voltage increases, you just crank up the clocks in the ATI CCC.
http://www.xbitlabs.com/articles/video/display/radeon-hd5850.html
To answer your concerns: ATI cards are more driver heavy than Nvidia. They usually release a new driver each month, the last being 9.12 (12 meaning the month). This doesn't mean you have to update every month, but sometimes there are compelling reasons. I usually check out the www.tweaktown.com website as they usually have an AIT driver analysis with some of the latest cards and performance increases or decreases. The ATI drivers allow you to easily overclock as well. You just unlock the overdrive tool via a button and move the slider. If I bought a 5850 instead of a 5870 I would be overclocking this thing day #1.
Another note: I see you mentions you have been an Nvidia customer for quite some time. I've seen many people in your shoes lately because ATI has their new shiny cards out and Nvidia has yet to do anything. It puts you in a hard place where you want to upgrade and you have been loyal to the green team, but the red team has all the shiny new stuff out right now.
Tweaktown 9.12 analysis page: http://www.tweaktown.com/articles/3074/ati_catalyst_9_12_windows_7_driver_analysis/index.html