I’d say the 980 (non-Ti) is neither here nor there. At the moment 980 Ti is the one to beat, with the GM200 GPU you should be able to start making a dent in 4K and VXGI, Gameworks, etc.
Personally I’ve seen all the arguments for the 970 4GB being good/bad/ugly but I’m just not feeling it. It’s got a good price point but I reckon the GM200 GPU is going to see you through the next few years.
If one is just upgrading GPUs then I’d certainly advocate for the 980 Ti. If one is building a new rig I’d also advocate for the 980 Ti, you could make up for the cost by getting a Haswell i5 (unlocked, of course) with stock fan and basic 16GB DDR3 RAM.
A Kaby Lake with Nvidia Pascal is about a year away or so but Haswell i5 with 980 Ti seems nice. For those with the budget of course go for the Haswell-E six core, DDR4 RAM and Nvidia 980 Ti.