ATi right now. The Fermi line was a catastrophe in so many ways. For the price, you got about the same performance as nVidia's ATi counterpart (or worse) plus extreme power consumption and heat production. If comparing the first Fermi line (4xx) with ATi's 5xxx series, ATi wins this battle hands-down.
Even right now, with nVidia's refresh of the Fermi line with the 5xx series, ATi still has a great chance at beating them again when the 6970 and 6990 come out this winter/early next year. As of right now, the best values in video cards are either the relatively low-end 5770, or the mid-range 6850. If you're looking for high-end then price probably isn't an issue, and you'd probably choose to crossfire/SLi anyway, in which either company would get you good performance (but again, ATi has power draw and heat nailed down, and the 6xxx series' crossfire scaling is incredible).
Oh, and the goodies: nVidia has PhysX, ATi has eyefinity. IMO PhysX, while adding some cool effects sometimes, is still a gimmick that isn't worth the loss in performance, while eyefinity with multiple monitors is immersive like nothing I've ever seen before.