I'd just like to correct an earlier post - 2 970s in SLi are not better in 4K than the 295x2, it's actually the other way around.
1 monitor with up to 2560x1600, the 2 GTX 970s rule. Most people will probably use this
For multiple monitors or 4K, the r9 295x2 is better.
So it really just depends on your configuration. But for the average (even hardcore, but average setup) gamer, or a visual designer, the GTX 970 SLi is the way to go.