Even though the 780ti can, at times, beat the 290x I beg to differ:
http://www.tomshardware.com/reviews/radeon-r9-290x-hawaii-review,3650-10.html
http://www.kitguru.net/components/graphic-cards/zardon/amd-r9-290x-review-part-1/15/
http://www.eurogamer.net/articles/digitalfoundry-radeon-r9-290-290x-review
"Moving up to 2560x1440, the wider memory bus of the dual 290 products becomes more important, putting the GTX 780 under further pressure."
Honestly, the $200 extra price mark really doesn't justify itself.
The R9 290x has more VRAM so at 1440p where VRAM matters, the 290x is a better choice in my opinion. Also, because of its higher VRAM it will be better in the future because games seem to suck VRAM up and even though demanding games at the moment average 2-2.5GB of VRAM usage, they get close to 3GB, so in the future this could be your limitation with the 780ti... Also, with mantle being integrated into multiple games, it will have a definite dominance over the 780ti in the future...
Why pay $200 more for similar performance (or worse) and less VRAM?
I'm aware that nvidia have certain features that a lot of people like, but that really doesn't justify the massive price gap between the cards. nvidia clearly overprice their cards... take the R9 295X2 and the Titan Z for example. The Titan Z is twice the price of the AMD and yet has similar performance.