Not completely convinced of Ngannou yet... But he has momentum and power. Overeem has a glass chin. Ngannou stoppage inside of 1-2 rounds.
While he's looked good so far this is a huge step up for Ngannou; he's fought nobody of note except a shot Arlovski. He's also fought nobody with the striking prowess and power of Overeem. I'm cautiously picking Overeem by KO.