I think Diamondback would win, but he'd have to work hard for it. Marv is a tough SOB to put down. But Diamondback hits like a truck. Literally. Luke totaled a speeding SUV by shoulderchecking it, walked away from explosions, falls from great heights etc., but hard hits from DB were sending him flying and causing him visible pain.
Marv is a WAY better character though.
__________________ Never take life seriously. Nobody gets out alive anyway.
Diamondback was at least keeping up with Luke Cage and hurting him/sending him flying with his punches until Luke stepped his game up. Luke is head-and-shoulder above anyone in Sin City's universe to the point where he could solo everyone given enough time and planning, and DB was rocking him for awhile. So ya, DB wins vs Marv.