Rainpaw :
If you're thinking of upgrading to the better, then I would go for the new 1080ti. It's 35% faster than the 1080 and it's also cheaper to produce which means it cost less than a 1080. To see more info about the 1080ti you can check out LinusTechTips in this video link. https://www.youtube.com/watch?v=L34ZkAF9q9w
1) Uh, the GTX1080Ti does not cost LESS than a GTX1080.
2) Is it worth an upgrade?
Well, it depends what games you play now. Do you need the FPS or Visuals to improve?
(If you get nearly the maximum quality and maintain 60FPS or whatever your goal is then I would not bother upgdating.)
3) Performance benchmark:
https://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_1080_Ti/30.html
GTX980Ti vs GTX1080Ti - 65%
GTX1080 vs GTX1080Ti - 84%
So the GTX1080Ti is about 50% faster than your GTX980Ti, however the GTX1080 isn't a big update. Again, many games you have probably run great already. It would be a waste for DOOM as that runs great, but look through the GAMES in this article and you can see the FPS you get for Ultra (or close) settings.
4) ASUS STRIX - that's my top choice for a GTX1080 or GTX1080Ti.
(I bought an EVGA. I fixed the thermal issue but the ACX3.0 fan or fans make a SCRAPING noise when turning on and fan noise at about 600RPM so that kicks in with light gaming. SIGH. Not sure how many are affected but if feels like a fan design flaw not quality control.)