If they box now, it wouldn't tell us anything about a fight between them prime vs. prime would be, in boxing or MMA. Also, while Mousasi has had a lot of fights, only a small percentage have been against Top 10 guys in his weight class who weren't over the hill. Almost everyone he fought in the first half of his career were no-names (and a few old guys). Contrast that with Anderson. Even after losing his belt at age 38 he's fought Bisping, Cormier, Brunson, Israel, Cannonier and Hall (who beat Mousasi once). I'm quite sure he has more mileage due to fighting more high level guys. I don't think Mousasi took much damage early in his career against the likes of Chico Martinez, Stefan Klever and Andre Fyeet (all quick 1st round wins--there were many others like that).