Probably this. While Fedor would always have a chance due to his massive power and Wilder's shaky chin, MMA guys have typically not fared so well in pure boxing matches vs pure boxers. If Wilder used his head and boxed, he should win comfortably. If he got into a swinging brawl, Fedor might just Arlovski him.
Wow a ridiculous hypothetical matchup where most keyboard guys are favoring Deontay! Damn its about time! Usually in order to sleep well at nights all the keyboard guys type their typical "Deontay's glass jaw gets exposed inside 3 rounds" fairytale despite the fact that Deontay hasnt been close to losing yet as a pro.