Well same macros, same calories, I'd think there would be shit all difference for muscle gain, but the thing is, macros in those types of food aren't the same anyway. Eating clean and not clean would probably change your average macros just by the types of food you are eating.
The junk food guy would also be getting more of his carbs from sugars, and more of his fats from "the bad fats".
The junk food guy might feel shit from eating junk food all the time, and that could affect his workouts
(I never feel that great after junk food)
It wouldn't be too bad for muscle gain, might slow fat gain though I'd think. Just eat clean, and don't worry about when you stuff up too much.