Should I sell my 980ti for a 1080 or buy another 980ti to sli for $400 CAD

Solution
Personally, I'd say, unless you have money burning a hole in your pocket, stick with what you have. I don't think the performance increase would be worth the cost of any upgrade.

-Wolf sends
What monitor do you have? What's the resolution specifically? The only reason you should be looking into upgrading to SLI 980Ti or a 1080 is if you are upgrading your current monitor to a higher resolution (like going from 1080p to 1440p or from 1440p to 4K).

Also 980Ti in SLI is about 15% faster than a single 1080, whereas a single 1080 is about 25-30% faster than a single 980Ti.