I have a 980 (2) in my box I use for AH, it works ok, 980ti is a BIT faster, but I think you're best bet is wait a couple months. This month's show nVidia/AMD are releasing looks at their new cards, the "1080" or whatever and Polaris/Pascal stuff and HBM memory possibilities too. Anyhow, prices will come down for the 980 by summer or even late spring most like, plus there will be new card options from probably BOTH nVidia and AMD now.
A couple months is well worth waiting IMO, as a:AH3 will just be up and going really most like, maybe even just starting, and b: due to that "no rush", well worth waiting for the new cards/prices, or at least the price drop on the 980/ti and AMD current cards which are much improved too.