If you really can sell your current GTX780Ti for USD300 and if a new GTX780Ti will cost you up to USD400, I think, selling your current card and buying GTX980Ti is the wiser choice, although you should pay a wee bit more.
With GTX980Ti, you are leaving the option to go 1440p or even 4k widely open a.k.a you can upgrade anytime you want, even if you do not have such plan today.
A single GTX980Ti consumes a lot less power than 2xGTX780Ti, if this is a concern to you.
A single GPU is than as usual free of all SLI's potential issues.
GTX980Ti scale quite a lot better (2xGTX980Ti has up to 80% more performance than a single GTX980Ti) on SLI compared to GTX780Ti (up to around 50%).
If you have the money, going 1440p 144Hz 27" IPS Monitor is incredible, I glad I bought the Acer XB270HU and now already thinking to get a second GTX970. In the next months, there will be more of those monitors from other brands plus the coming super monitors like ASUS 3440x1440 75Hz 34" IPS curve monitor. I would perhaps sell my current XB270HU and go for that ASUS monitor.
Yes, GSync is not important and does not really worth the premium price at the moment but any of those fast IPS monitors are really a nice upgrade to any 1080p or 1200p monitors. Yup, those kind of monitors are expensive even those without GSync but the prices should go down quite a lot in the next 1-2 years. You would really want to start planing for a monitor upgrade in the next 1-2 years.