I will get specific here. Rodman when he was young was simply an incredible defensive player and could guard anyone from shooting guards to shorter centers. I am a guy who is a big fan of technique and Rodman would have been a great teaching video of how to guard. His ability to withstand pain, his ability to move his feet, etc are not matched by anyone including KG. However at the specific PF position, KG might have an edge because of his short blocking and team defense ability. So if you ask me who the better defensive player I would say Rodman. If you ask about guarding the big men at the power forward position, I would say KG.