It was El Puma who nailed this perfectly for me. He once said the most gifted fighters to live where Howard Davis Jr and Julian Jackson, because you can't train any one to have tornadoes in their fists or lightening in their punches...at least thats what I think I remember him saying at some point. Classic forum I think...
Julian Jackson had the kind of punching power that comes along once in a century. For a guy at any weight to consistently put guys to sleep the way he did was outrageous. On the flipside, Hagler had the kind of chin that comes along very infrequently. Hagler ate bombs from Mugabi and from Hearns and was never truly in trouble. In the end, I think Marvin wears out Jacskon H2H and beats him in a 5-6 round fight max. Hagler might even touch the canvas on this one, but still wins a TKO.
Eventually got detached retinas in both eyes as a result of his career. There are few things more beautiful to watch than a Jackson KO. However I think you can count me in the group that thinks that Hagler would have been able to stand up to it.