r9 290 best value for your money: 400 dollars you can find it cheaper on used in amazon
gtx 780 beast of overclocker: 500 dollars
r9 290x: second best single gpu on the market: 550 dollars you can find it cheaper on used in amazon
I would go for the GTX 780 SC. It is one of the best cards you can get with that budget. if you don’t really care about the extras you get with NVIDIA, the R9 290 is also a great card. They are both very close in performance, it just depends on what games you will be playing.
Watchdogs is performing better with Nvidia cards due to Nvidia Gameworks ( basically the programmers at Nvidia work with the company's developers to optimize the game for Nvidia cards) however the difference in fps is maybe around 5fps in Watch Dogs, not too bad.
I just bought a EVGA GTX 780 FTW edition for $525!
it is faster than the superclocked version and has dual bios for some sick OC'ing!
After i get it and OC it even further, i would be willing to put it up against any 290X