Smash's characters look better, and MK8's environments look better. That's what I think.
While Smash has SOME beautiful environments, others look noticeably worse. Best example being MK8's own Mario Circuit:
The Smash version has blurrier textures and is much more polygonal (dem trees). Lighting is also inferior.
Of course the Smash version will look fine in motion, and the downgrade makes sense, a you cannot get up as close as you can in MK8, so it doesn't need to be as detailed (plus it helps performance). But some of the other Smash stages look tons better (Battlefield, Final Destination, Mario Galaxy, Windy Hill, etc). While in MK8, the graphical quality is very consistent, with no track looking more or less polished than any other one (this applies to the upcoming DLC tracks too).