At this point, I'm favoring twin 1070s over a single 1080. But the only advice I can give you at this point is:
1. Stay away from the Founders Editions.
a) it doesn't matter which one you buy as they all the same
b) All FE cards have been exhibiting thermal throttling
2. As to which one to buy right now it's hard to say ...
a) I like to avoid purchasing the 1st round of cards as prefer to let the bleeding edge folks get stuck with all the problem stuff and wait till manufacturers have ironed out the early problems... case in point EVGA SC 970 where 1 of the 3 heat pipes missed the GPU.
b) If we go by the last 2 generation (7xx / 9xx) cards, and here I am talking about the "mainstream cards", I'd have to rank them MSI Gaming > Gigabyte G1 > Asus Strix > EVGA SC. Before that, I had Asus at the top thru 5xx and 6xx series.
c) There have been no good tear down reviews and the non-reference reviews are just coming out now. The only comparison reference I have seen so far is this here:
http://videocardz.com/60838/msi-geforce-gtx-1080-gaming-x-is-much-better-than-founders-edition
NVIDIA’s own reference design suffers from severe throttling just after few minutes. It probably wouldn’t be that bad if not the frequency spikes. While average clock is somewhere around officially stated boost clock, those spikes cause micro-stuttering, which negatively affects gaming experience.
Hardware.info:
Founders Edition suffers from a horrendous amount of throttling and it runs +- 150 MHz lower all the time.
Meanwhile, MSI GTX 1080 GAMING X generates almost a straight line for GPU frequency (~1910 MHz), with no spikes and rather constant sub-70 C temperature. This should mean that the gaming experience will be much better, and card should theoretically generate better results in most tests. Also according to H.I. this is also the best custom design they so far tested.
No while that might seem like a nice endorsement ... we don't exactly know how many custom designs they have played with and which ones they were.
Remember like sports teams, no one stays on top for long ... nobody likes coming in 2nd ... or worse .. and those folks tend to work harder to get to the top while the ones at the top tend to get complacent.