2xGTX970 SLI is around 10% faster than a single GTX980Ti but limited with the 3.5GB VRAM, could suffer from all SLI issues and consumes more power.
Some words about the GTX970 on SLI,
Until now, I see relative 0 problem with 2x GTX970s on SLI regarding the stuttering, etc. but my previous experiences with SLI still said that the usual SLI issues could still exist.
GTX970's 3.5GB VRAM could be a big problem if you plan to play on resolution higher than 1440p e.g. 4k.
Remember 2x3.5GB VRAM on SLI is 3.5GB VRAM and not 7GB VRAM.
If you have already a GTX970, getting another one is the best solution. This is easy to decide.
Personally, I am considering to get a second GTX970 for me.
If you do not have a GPU yet, well, you must decide either 2xGTX970s or a single GTX980Ti.
I would prefer getting a single GTX980Ti.