the R9 290X is clearly the better card, highly recommend that card over the 780. I had same choice and took the R9 290X. Extre 1GB GDDR5 Vram, wider bus, all making it have the better potential at maxing out games for longer and handle multiple/high resolutions far better. the R9 290X as reviewed crushed the 780 in 4k.
http://www.eteknix.com/4k-gaming-showdown-amd-r9-290x-r9-280x-vs-nvidia-gtx-titan-gtx-780/
Check that out, even though you may not be going 4k any time soon, this is a good indicator of it's performance to come.