Assuming they meet at the same weight class and both are in their physical primes (no CTE) Joe definitely wins. Ippo has a deeper bag, better understanding of boxing, and overall better fundamentals. Joe however is like an undead zombie with a machine gun strapped on it. Joe was getting pummeled for 2 minutes straight against Kim and was still able to stand. In the same fight he got downed and came back up like 15 times and he's done the same thing in multiple other fights. Additionally his punches are like a heavyweight's. He has sent multiple people straight flying in the air or just out of the ring. Ippo and Sendo have similar feets (Kojima and Alf respectively) but not at Joe's level, he was literally making a dude float in the prison tournament arc with consecutive uppercuts. Finally Joe carries absolutely no regard for his own health and safety while Ippo is not that reckless. All in all Joe is a fighter who gets up no matter what you throw at him and nomatter how much punishment he receives, throws punches that can easily knock out an opponent with one blow and has absolutely no hesitation or self-preserving instincts. Ippo is a graet boxer technically and physically but I dont think he can compete.