Raw GPU power and I find it can depend on the game engine/game as well as freesync or g-sync. I am playing far cry 3 on Ultra and find it will happily render at 100fps, then drop to 60fps, then into the 50s and ending up in the low 40s. So even though I am using a g-sync screen I will still get some occasional stuttering. No tearing though.
So I turned down MSAA to 4x and now I get some occasional MINOR stuttering. On the other hand I was playing dishonored 2 with everything maxed out and it is buttery smooth.
I think I mentioned it earlier but I am playing @ 3440x1440. I just couldn't go back to a 27" screen (2560x1440) or a 34" (2560x1080) screen even though I know I am going to get more consistent performance across games. I did however buy the most beastly GPUs I could get my hands on. So I am in with a fighting chance.