I like Cormier better, he is one of my favorite fighters, but I would actually favor Overeem. DC struggled with Gustafsson who almost KOed him, Overeem has better striking, power and grappling than Gus plus is bigger so I'm afraid he would be too much. He's better than Gus everywhere IMO except for his chin/defense and cardio but DC doesn't have huge power to exploit the former and the latter might not come into play.
I mean, DC can wear him out or even shatter his china but I'd go 60/40 Demolition Man in the end.