It's a tough one guys, tbh, the 780Ti is a fantastic card, there is no doubting that, it's insanely fast, quiet, looks awesome, a perfect card really, the biggest issue for me is the cost.
The REF 290X is also insanely fast, but can be loud over a certain fan speed, normally 55-65% depending on user and doesn't have the aesthetic appeal like the 780ti (although after getting my first i was quite impressed actually by how solid it felt and looked)
In terms of Pure Performance the 780Ti should on paper be the better card, but what i found was FPS at 1080P were no better than my 290X, infact in BF4, hitman and sleeping dogs, the 290X had a slight lead with both cards at stock.
Crysis 3, Skyrim and bioshock all favored the 780Ti, with it beating my 290X in all of these games.
Overall i thought that while the 780Ti probably/is the faster card, it doesn't actually show it in games, at least the one's i have, certainly not enough to justify the cost anyway.
Please don't take what i've wrote as nvidia slating, it's not, just my honest opinion on two fantastic cards, i just found myself looking for the performance to match the price difference between the two and i couldn't, at least not in my system.