Both have their merits in my opinion, 27" is where 4k gets pretty useful I think since the pixels are otherwise larger. A single 780ti won't get 60fps on max settings at 4K and it won't get much, if at all, over 60fps at 1440p, so either way to get the benefits you need to SLI, and if you don't you may as well save your money and get a 60hz 1440p as you won't see a difference.
http://uk.hardware.info/reviews/5419/10/nvidia-geforce-...