nVidia for me personally.
I have used both in different systems plus ones I support. Yes 3DFX was bought by nVidia and they are just now starting to introduce some of that technology they got from them, 3dfx was way ahead of the rest. The ATI cards have come along way and yes they still can have support issues, that you tend not to see in nVidia. On the other hand ATI usually is more moderatily priced between the two. Both are solid performers for games, and windows usage. The winner in the more Graphics market is nVidia, and this will be for some time to come because of the infusion of the 3dfx technology they bought.
Now if you are just talking about graphics there are better cards than both these guys out there, especially for CAD, CGI and other very high graphic programs, most of these cards start in the 1000 dollar range and go up from there.
For gaming either will work, as ATI works better for some games and nVidia for other games. Look for the best preformance for the game you play verse price and one of these guys will stand out better than the other. It really comes down to what games you are spending time playing on-line, and cost to decide what card to get. As for SFC either will work just fine, and in the lower end card as well.