R9 290, especially since two R9 280X is quite the overkill on a 1080p monitor. And single-cards are much more hassle free since:
you can't expect 100% performance in all games when you play in CF, it's much louder, much warmer and more power demanding going CF.
R9 290, have this gpu myself running 1080p, you'll hit Ultra in 99% of all games. You CAN go R9 280X CF for a higher FPS, if you want all the problems it brings.