On paper I think the 290x comes out better but sli is more stable than xfire, also, cooling and noise would be a constant issue, plus I'm biased towards nvidia for driver support.
In my opinion, the extras you get if you go Nvidia (low heat output, low power consumption etc.) completely justify the higher price.
The 290x will probably perform better at 4k than the 780ti due to it's 512 bit memory bus but even then it would only be by a couple of fps. It would also depend on the game. Every other time the 780ti would either match it or beat it.
I would go Nvidia especially after the failure AMD had with mantle.