I think the difference is quite big. You are definitely getting a smoother experience and higher resolution if you go with GTX 1080 Ti. I just ordered a GTX 1080 Ti and is hoping it will drop today. I upgraded from a GTX 980, which is a bit weaker than 1070, but pretty comparable. I’d say go with the 1080 Ti if you can afford it.
When using vorpx, you should be able to supersample very well…totally worth the upgrade. The extra horsepower will enable you to do much more tweaking.