I am going to throw my thoughts out there and hopefully help you make the best choice.
I went from using a single card for years, to about 2 years of SLI, and now back to a single card.
My memory may serve me incorrect but I think I first added another 670 and was blown away by the increase in FPS. It really is a brilliant tech... when it works.
Over the years I became increasingly more annoyed with it due to lack of support for new titles, having to fiddle with settings, profiles, having to use NVidia inspector to get titles to work, and then more often than not it just not working. Added to that extra noise and heat I had enough last year and sold my 780SLI for a 980ti. Could not be happier, best choice.
If you don't mind experimenting and potentially fighting with settings and profiles for ages to get your games working, then go SLI. If however you value just playing the games and not having to think if your next anticipated title will support SLI then go for the single fastest card you card afford!