We don't really know who had the better chin, but Hagler's chin was more proven because he fought some insane punchers who hit him with some big punches. I suspect there is no meaningful difference but it'd be nice ot see Hearns or someone like him tee off on Golovkin before making that call. Power, I'd probably give the nod to Golovkin on a raw basis but if I was guessing who hit harder on the third and fourth punch, I'd go for Hagler. Again, that's obviously not proven and Golovkin could swat.
Wouldn't say Golovkin's chin was better than Hagler's, but probably about equal. Both could take tremendous punches without taking a step back. As for power, I'd say Golovkin was more heavy handed. Always hard to compare the same day weigh in era with the day before era, though. Golovkin was typically around 170 lbs in the ring at MW, while Hagler weighed in with pounds to spare on some occasions and probably didn't rehydrate very much.