I just want to point out as I am not sure if people here realise. You can't compare MHz for MHz. Just because the 970 is several hundred MHz higher, it doesn't follow that it is therefore superior to the 1150 of the AMD card. There is way more to it than that. The 390 has more of a lot more other things that are important in a graphics card. Both cards are blocked higher than the 980ti but are obviously inferior.
You can almost never compare MHz to MHz in anything unless it is nearly exactly the same. It is a trap I fell into myself and now I realise that I do not really know enough about electronics.
Another thing. You can't just rely on one analysis. Some analyses are flawed as well and with all due respect to these people, a lot of the reviewers do not carry it out in a scientific way. I saw an analysis which decided the 390 was better but there many be other ones as well which contradict this.
They perform very similarly. WIth regards to crossfire/sli, if I wanted to, I wouldn't crossfire/sli either of them for the following reasons. I would want to SLI two 970s if it wasn't for the 3.5Gb RAM. I have no idea if that will be enough for 4k. Naturally that would lead me on to the 390 as it has 8GB RAM. I have an 850W PSU but a lot of people wouldn't have the PSU for these. But the real problem crossfiring the 390 for me is the noise. I have a 390x, it is so noisy that the only reason I am not unhappy with it, is that I always have headphones in. But with two of them, it will seriously sound like a washing machine, its that noisy. I wouldn't crossfire them for that reason, it would just ruin the gaming experience even with headphones! I am happy with my 390x but it is at the limit of the noise level I can tolerate.