ChrisR83 :
I would go for the 780, the 290X is slightly more powerful, but @ 1080p you are talking 5-10fps difference depending on the game, if you are playing on a 60hz monitor, you monitor is only going to display 60 of those frames so there is no reason to be running 100+fps. also with nvidia you get shadowplay, geforce experience, ect.
http://www.anandtech.com/bench/product/1056?vs=1036
later on, the R9 290X will benefit, currently, no. The OP may want this card to last the longest it can be, and in a few years time, the R9 290X will be defiantly pushing to get 60fps ultra 1080p, i doubt with some games next year it'll be getting above 60FPS ultra.