pretty much comparing apples with oranges imo
But gonna answer anyway...
Overeem is a more accomplished striker
K1 champ , Dream mma , Strikeforce belt... All at the same time....
His knees are deadly, he is a very experienced striker, probably the most experienced fighter in the heavy weight division in the UFC imo.
Conor is flashy, very fast and very accurate. He is very good, but I think Overeem has more tools & tricks and (used?) to be a very intelligent fighter imo. Overeem is a far better fighter than Conor imo.
So it comes down to:
More experience
More tricks & tools in his arsenal
Very powerfull
Experience against best of the world
VS
very fast and accurate striker who is a bit too big for his division, and frankly has little experience and did not fight the same level of competition.
Take your pick.