Like the common saying goes, 1 card is always better than two.
HOWEVER
Speaking generally, a GTX 970 gains around a 7 - 10 frames more than a RX 470 (this is referring to various game/software benchmarks I have seen in my time which of course are not always accurate).
Two GTX 950's edge the compute performance of a GTX 970 which means they will beat a single RX 470 by quite a chunk.
The main advantage the RX 470 has is the VRAM which has 4GB over the 2GB the 950 SLI has.
Two 950's will have a larger power output having a bigger impact on your electricity build and finally how well do you know your computers? SLI can be easy to set up but a pain to maintain regarding temps etc.
So take your pick, more power or less stress?
Hope...