It's very close. The longevity is about dead even both in the range of 80+ fights. Overeem has more names on his resume with Cro Cop having more titles, both won a K1GP though Reems run was much more stacked. Both were very strong finishes very rarely winning by a decision, Reem was more well rounded as both a grappler and striker, but he had the worse defeats and was finished a lot more.
I'd give the slight edge to Cro Cop, their kickboxing careers were pretty equal though Cro Cop was better in his prime and competed in the glory days of K1, he won more titles in MMA as well as being a little more consistent. As talented and respectable as Overeem was, his prime was clearly highly influenced from being steroid abuse. I doubt Cro Cop was natural himself but he wasn't clearly abusing performance enhancing drugs like Overeem.