You will win fps benchmarks with GTX780ti SLI, but will have some gameplay issues.
Some games do not support dual cards well or even at all.
You will avoid stuttering and tearing issues.
At the same price, I would pick the GTX980ti hands down. It will handle a 4k monitor decently.
My suggestion is to buy your 1440P monitor and see how you do.
OTOH, 2 used GTX780ti cards sold on ebay will get you most of the way to a new GTX980ti.