I've owned amd cards and Nvidia cards throughout a few generations. On the amd side: Hd4670, Hd6950, Hd7970, R9 290. On the Nvidia side: Gtx 460, Gtx 780, Gtx 980 (current).
The one sticking point that pushes me towards Nvidia is opengl performance. It's been a problem for as long as I can remember and I'm not sure Amd has actually done anything about it beyond a few placating words to gamers.
The most recent game I had trouble with was Trine Enchanted Edition, which I believe runs on Opengl much like the dark mod. Going from the R9 290 to the Gtx 980 was a night and day difference, far more than the average difference in fps in Directx games. It basically went from an unplayable 30 fps at lowest settings on the R9 to nearly 60fps on high settings on the Gtx980. All the while the improvement in other games was marginal at best.
I haven't played the Dark mod in awhile (Thief fm backlog), so I'm curious to hear the experience of other users with Amd cards. I think the last time I played tdm was with the Hd7970, and I had to turn down a few settings (mainly AA) to get a smooth 40+ fps, I seem to remember maxing it out on the Gtx 460 back in the day.
Let's get a discussion going here and if I'm not crazy/other people are having similar issues, maybe we can sticky it this time to let Amd know it isn't appreciated.