Pros of a single 980:
1) You have a faster single GPU (for some applications, that is better)
2) If case size/space, heat, or power is a factor for you, a single card wins
3) You will have the option of adding a 980 in SLI later if your heart desires
4) $130 cheaper
Pros of 2x970 SLI:
1) More total GPU power, which will be better in many applications/games
I bought a EVGA 980 superclock but have not built my new system yet (other parts enroute). For me, I have a small form factor build, and although I could put two 970s in my Sugo SG10B, I prefer having the lower power and heat for now while still having the option to add a 980 later AFTER I experience the performance of the single 980, which might be good enough anyway.
I suggest...