Hi there. I run a 480 crossfire and a lot of my friends have a single 1080. Here's some of the pros/cons to the crossfire:
Pros:
It would be less of a cost to upgrade
You will see very similar performance in games THAT SUPPORT CROSSFIRE. Many even popular games do not!
Cons:
GPU market is very high right now, and the 580's are very overpriced
Lots more power consumption, need correct cabling and PSU for it.
You need a board that supports it and runs at least x8 by x8 -- no x16 x4 BS.
Many (even AAA) titles don't support it, so it's useless in some scenarios.
My biggest complaint with the experience is the lack of Crossfire support, the occasional Crossfire glitches in games like Witcher 3 and PUBG (was unplayable on Crossfire), and the heat it creates. Even having custom cards, identical, in a mid tower, I can still see 80+ degree temps on the top card.
I'd recommend the 1080, despite my AMD fanboyism.
Though, I do enjoy Freesync, and I believe Freesync has a better future than GSync.