Overall I'd suggest that the more GPU power you can throw at something like the Rift etc. the better, if only as SS & graphics settings in apps can be upped.
On default settings, the image quality should be similar or the same (though some games will auto-adjust the graphics so 1080 Ti will look better). The bigger difference is that the 1080 Ti should run at full 80fps but 1660 Ti will probably be running at 40fps with smoothing. Honestly, I think this looks fine but some people have said it causes artifacts.
Actually, now that I am looking at it. A 1660 Ti would probably be fine. If you want to save the money it will probably still be a great experience (meeting recommended GPU). Obviously the 1080 Ti will be faster/better but you might not even notice a difference in many games.
