Different gameplan (against a slightly lesser version of Louis than the very best incarnation) which could not be implemented successfully by Marciano. Schmeling hit his purple patch in that fight as a counter-puncher --- that just wasn't an option for Marciano. Not against lesser men, but definitely not against Louis' speed & reach advantages. If Marciano beats Louis, it has to be by another way. It is too facile to imply, "Marciano is greater than Schmeling, therefore, Marciano could win." Personally, barring a vintage, Walcott-esque one-shot KO from Marciano (which I won't discount), I see no way around Louis' offensive arsenal for him. I mean, I look at it this way --- when one guy has an edge in height, an edge in reach, much faster hands, far better accuracy, infinitely superior combinations & punch-variety, & a magic KO punch from either side, how's the other guy gonna beat that? You need to be more than special, or have a history of overcoming the odds. Marciano wasn't good enough. He'd be devastated by Louis' offense, & to those who occasionally appear to posit Marciano would deal well with being hit cleanly & consistently by Louis, I say not in this lifetime. Louis in seven. Either the unthinkable happens & Marciano takes the count (much more reasonable than many would imagine), or his corner don't let him out for the eighth stanza. Either way, I think Louis takes him apart with clean, precise shots on the way in. Louis would lay some cringeworthy uppercuts on his man for the highlight reel.
I think Louis was a better fighter post championship than he was pre-championship. Rocky could take Joe out but if Marciano could get dropped by old blown up Lightheavyweight Archie moore..Louis could put him into LaLa land. Gimme a prime Louis to win a decision. both men are dropped along the way.
In all honesty I think Louis wins quite easily. And in the end absolutely brutally. Hell make it the Louis who beat Wallcott and I'll put money on him.