I'm going with Joe for similar reasons to ones I've explained before. Hagler was a 160 figther from same day weigh ins, Joe 168 from day before weigh ins. Hagler would be going up effectively 2 divisions to fight Joe and he simply isnt proven above 160 whereas Joe at 168 has the best record at that weight bar none. If Joe had been fighting in Hagler's day he would have been a light heavyweight. I think the step up would have been a little too much for the marvelous one and he would have lost a close competitive fight.
Being put down is insignificant. Especially for 3 secs (Hopkins) or 4 secs (Jones). Ali, who was regarded as having a great chin, was put down as many or more times than Calzaghe, and as he said: "Nothing wrong with getting knocked down, so long as you get right back up again." Calzaghe was KD'd by Mitchell and he got up and stopped Mitchell in the same round. Imagine what a prime, much smaller, Hagler would do ? I already have.
I think this is a terrible match-up for Joe. Hagler had great stamina, more power, indestructible chin, better technique than Joe does. I would say Marvin is a 2:1 favourite here.
Don't forget, Hagler ate Hearns best punches in round one of their war, and far from being weakened - let alone KOd like most of Tommy's opposition who got hit flush - it actually spurred him on and increased the ferocity of his attack. Hearns was one of the hardest ever hitters at middleweight or below, KOing durable light-heavies into his 30s. Hagler had the best P4P chin in the history of boxing. Just as James Toney was durable at heavyweight, Hagler would have been too. Only the real hard punching heavies could have KOd Marvin. Calzaghe doesn't even have great power for super-middle. There is no way Hagler is even getting wobbled in this fight. The same cannot be said for Joe, who was floored by 40something Hopkins, not exactly a banger, and a 39 year old Roy Jones.
Does Joe punch as hard as Thomas Hearns? Hagler ate Hearns best shots and shrugged it off like nothing.
Hagler by late TKO in my opinion, he would walk through Joe and grind him down. Would be a great fight though.