SLI or just one

Solution
The key advice in any build scenario is when purchasing a discrete GPU, get the single most powerful GPU you can afford and be done with it. Even if you could SLI, the titles that take advantage of them are slim while productivity apps that can take advantage of the feature are even more limited.

If your budget allows while the rest of your system's specs are compatible, then go for the GTX1080Ti.
The key advice in any build scenario is when purchasing a discrete GPU, get the single most powerful GPU you can afford and be done with it. Even if you could SLI, the titles that take advantage of them are slim while productivity apps that can take advantage of the feature are even more limited.

If your budget allows while the rest of your system's specs are compatible, then go for the GTX1080Ti.
 
Solution
It actually highly depends on the game you play.


See some games are optimised for multi GPU use, while some other games are NOT optimised for multi GPU use.
Such games that are optimised for SLI/Crossfire are like Tom Clancys Division

Other games like the Witcher 3 aren't optimised for multi GPU use.

So for the games that are optimised for multi GPU use, you could consider the 1070, as it does yield better performance, but you'll have to decide whether you can live with the cons of SLI.

Pros of SLI: you get better performance for paying potentially less.
It makes your system look better I Guess? It's just an opinion thing. IMO, it does : D

Cons of SLI: You get to spend potentially more for lower performance than just using a single card(depends on game)
You might end up with either a louder system or a hotter system or worse, both xd
You'll have to get an SLI bridge (easy enough to fix)
SLI configurations do take up more power from your system. (2 1070s total 300W, a 1080ti is 250W)


yea

So if you can live with all the cons then go for it! Or if either option suits your current situation better


: D