I don't like getting into mma vs boxing stuff because its too subjective. Boxing is my favorite sport so I'm going to always think it's better. I think people feel the talent level in boxing is better because when boxing is done at a high level it looks "pretty" as the boy @Yuzo would say. Since mma has so many disciplines going on and it being such a young sport that even at the high level it can be perceived to be "sloppy" which makes dudes feel like the talent isnt there imo. I could be wrong though.
As far as which one is more legit is the guy training to box since 10 years old more legit than the guy who wrestled as a kid, moved on to bjj and eventually striking?
I just think lesser talent gets weeded out much quicker in boxing.