I would go with the 780 and if you feel down the road it doesn't cut it for 2560x1600 than grab another one. The price difference isn't worth the gains in my opinion.
I agree that extra 200 bucks is way to much for that little performance increase I went and did some benchmarks and guess what only a 7.4 fps increase on average that is almost nothing.So my solution was get yourself a gtx 780 lightning from msi or get yourself a evga classified I personaly own two lightning and man those things are epic and they have excellent overclocking potential...
If your willing to pay that much for a 780ti then you are much better of considering 2 x GTX 770 in SLI, much faster than a 780ti and also cheaper.
It will last you a long time and play anything you throw at it.