Difficult decision, 780Ti and 980 are very close performance-wise and 170 bucks difference for like 5% or so boost 980 offers over 780Ti makes it no brainer really - 780Ti is the better choice.
The wildcard here is the fact that 980 comes with additional 1GB VRAM and since you game at 1440p you MIGHT have a use of that.
Basically, the question is whether you can compromise a couple of FPS and be less future-proof, but save 170 bucks now.
I'd say - go for 780Ti, because 980 is really raw as it is... It's a product of Nvidia being late on shifting to a more advanced process, they pulled out a new architecture, but stayed on 28nm process, which is OLD now - this is what 980 is.
Seeing how AMD is preparing a new weapon of mass destruction soon, and Nvidia as well going to move to more advanced process at release an inevitable refresh of 980 pretty soon too (the REAL 980 and not the half-baked thing we have now), I'd say - go for 780Ti, so you spare yourself from banging your head on a wall in three months seeing your 980 crash and burn both in performance leaderboards and price.
So I'd take a very adequate and great-performing 780Ti and save those 170 bucks for a massive upgrade next year.