What's better?

I wouldn't bother to be honest. It would use less power, but you Norwegians bathe in oil anyway :) You wouldn't see a tangible performance increase to justify the cost.
 
Yes, you can SLI with GTX 970 - exactly the same way as you do with your 580s.
Even though I personally would choose the R9 290X, the GTX 970 is a more sensible choice. I prefer AMD to NVIDIA, but facts are facts :) You wouldn't be able to tell the difference between the two in-game, tho the 290X is a bit more future proof - but uses way more power and produces more heat.
Depending on SLI scaling (you never get double performance by adding a second card, best case scenario is 99% for select games), all three solutions should easily max out Assassins Creed: Unity.
 
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