Conclusion

Having a quick look at the results shows, that both configs are performing almost the same, although the two GTX 970 have a little advantage. At FullHD the 970 SLI is on average 2.8% quicker than the 980 Ti, at 1440p the difference is 15.7% and when testing 2160p we see the two cards be 6.2% quicker. Averaging the three resolution values results in an 8.2% advantage towards the GTX 970 SLI. Overall it might seem that the two configurations are very close to eachother but a quick look at the individual results shows that this highly depends on the game your playing. Especially at very high resolutions you will notice that Metro Last Light and Far Cry 4 benefit largely from the 980 Ti's 6GB VRAM.

Comparing the GTX 980 Ti with two GTX 970 in SLI is actually interesting. Games which are not limited by the 4GB of VRAM from the GTX 970 show consistently higher frame rates, but once more than 4GB of VRAM is used, the GTX 980 Ti pulls away. In other words this means that two GTX 970 combined offer more raw rendering power than one GTX 980 Ti, which is quite logical if you keep in mind that two 970's feature more CUDA cores, TMUs and ROPs. Nevertheless more VRAM can really work miracles when it comes to ultra high resolutions and very complex scenery. Overall these two configs are performing on a similarly and also the prices are close together - 706 Euro (980 Ti) vs. 648 Euro (970 SLI). But in the end we'd recommend to buy one GTX 980 Ti, since the 6GB VRAM will be able to handle future games better.