If you haven't bought any card yet, get 980. I have 2x 780 TI in SLI and I know for a fact that these will last for couple of years definitely.
780 TI - Kepler architecture
980 - Maxwell architecture
Just because 980 doesn't have as much memory bus, bandwidth etc. It doesn't mean that it will perform worse. Maxwell CUDA cores perform better than Kepler CUDA cores by 40% I assume, that's why there are less cores 2080-2880? (800 difference I believe ). 780 TI has 2880 CUDA Kepler CUDA cores and 980 has 2080 Maxwell CUDA cores. Maxwell doesn't use as many CUDA cores, therefore there's less power usage. 980 is only 5% stronger than 780 TI, and it uses around 100W less than 780 TI. Personally, it's issue that is connected to money. If you'd like to save up some £££, then I suggest to get 780 TI only if you have PSU that is sufficient for 780 TI. If you want something such as 980 get it, but sooner or later there might be 980 TI which will be far superior and even I (2x 780 TI owner) will have to upgrade if games will become so demanding (or programs etc).
It's your choice really, CPU might bottleneck your 980 in SLI (if you'll have SLI) so I'd reseach it 1st.
I own 2x 780 TI and 4790K processor and it's not bottlenecking.
Personally, I'd wait for 980 TI version (might not come out soon, but I guess it'd be worth the wait. I assume it will come out in 10 months. (same difference like in between 780 and 780 TI).
If you can't wait, get factory overclocked 980. Wait for all the versions 1st to see which one is best.