From what i saw Joe wins for the following reasons.
1 Ippo is strong but Joe still strong enough to throw a man bigger and heavier than himself over his head with just one arm, punch Danpei with enough force to send him flying a few metres through the air and smash a door off its hinges, ripped a punch bag with his punches,Ruptured the internal organs of Gondo Goromaki, and kill Rikishi with a punch in his temple.
2 Joe has more natural talent than Ippo he is capable of grasping most techniques quickly and adapting to various changing situations, in the first fight of his career he managed to scored a double K.O on Rikishi Who was already a professional and was capable of beat three bulls and was lightweight, while the first fight of Ippo he was beaten by Miyata.
3 Joe fights with brawlers all the time and i don't think that Ippo has a better stamina for what I know Ippo never fought a 15 rounds fight, Joe on the other hand did this even suffering punch drunk syndrome so i think Joe has a better Stamina too.
4 Ippo may have more techniques than Joe but none of that would work on him because his durability is insane, He endured punches of Rikishi that was strong enough to knock down three bulls and the blows of Jose mendoza that was capable of fracturing a human skull with one punch so the liver blow and the gazelle punch won't work and he could counter the dempsey roll since he has already counter the utterly unpredictable Harimau, whilst punch drunk , Ippo's dempsey roll would be child's play by comparison and even if he could not , he tanked Kim Young Bi's Chum-Chum(a seamless flurry of punches that killed two men of the two times it was used) so even the dempsey roll isn't a winning card for Ippo, and joe has a terrifying ability to copy the oponent's techniques.
5 Let's no forget that Ippo has problems with boxers who are good at counter punching , the fight with Sawamura and Gonzales proves that, and Joe is excellent with counters he managed to land the Triple Cross Counter (an extremely difficult technique to pull off) on Mendoza, despite being punch drunk and being completely blind in one eye.
6 As for strenght and durability, Ippo tying with Takamura in a arm wrestling match do not mean anything even because that's not mean he have the same Striking Strength, and Ippo whistanding a couple of punches of Takamura (with a headgear) before get ko'd still a inferior durability feat compared to Joe tanking punches of Rikkishi (with no protection) who was able to beat bulls (cows in the manga) and Takamura beat a asian Black Bear ( who weights in general 110 kg) with a inch of his life. Rikkishi beat THREE BULLS as TRAINING and (Bovinae weights around 720 kg to 1.100 kg) so yeah Joe's durability is beyound Ippo.
7 Joe has more willpower than Ippo , as already said in this thread Joe's fighting spirit is astronomical, his drive to fight has enabled him to defy all odds and push himself further than what any person could ever hope to achieve, while Ippo's determination was easily crushed when he got beaten by Date Eiji ,besides we know that Joe got excited with the news of Carlos Rivera and the strong man he was, There's no question then in a contest of wills that Joe wins.
8 Yeah Joe lost 5 times but unlike Ippo, in 4 of his loses he had a handicap against him 3 were stopped because every time that he punched a guy in ther head he vomited because of post-traumatic stress disoder that he got in his fight with Rikkishi. and the other was vs Joze Mendoza he only lose that beacuse he was already in advanced stage of punch drunk syndrome but managed to lose by decision, and he only lose by ko in one fight, that was against Rikkishi when he was early in his carrer and was very impulsive an inesperienced he lost because he couldn't get up in the count of 10 but he wasn't uncounsios unlike Ippo whose 3 losses where by KO.
9 And finaly For me the main reason why ippo would never have a chance against joe, is because ippo is so weak mentally unlike joe. Remember in the manga when takamura drew a line that separs the world level and the average level, for ippo after his fight against gonzalez ? Ippo got all scared and couldn't move forward pass that line, unlike joe, who would pass that line with full confidence and guts ànd manliness, or when Ippo became punch drunk once, he was ko'd by only 2 punches, on the other hand Joe fight against the best fighter in the world and go for a decision, that's an example like many others why ippo is mentally weak. So yeah for me assuming the speed is equalized, Joe wins with mid difficult.