We won't be able to tell at 160, because there's currently no middle in the world for GGG to face who has the power Gerald had.
I'd pick Golovkin on points. I think his offensive and defensive skills are better than McClellan's. Perhaps not by much, but better. With McClellan's power, it's not like you can be too comfortable picking against him versus any 160 lber, though. He'd win his share against anyone. Hagler might be the best pick against McClellan since his chin is iron and he figures to be able to outbox him.