I would argue that your position is determined based on who you can defend. With that in mind, I have much more faith in Flagg playing the PF position at the next level and functioning as a secondary rim protector (4.5 BLK%/3 STL%, 6'9" with a 7 foot wing span) vs Banchero's ability to play center and primary rim deterrent (2.7 BLK%/1.9 STL%, 6'10" with a 7' 0.5" wingspan)
Ultimately, I view them both as big wings - Flagg's is of the Grant Hill/Tatum flavor while Banchero's is of the Carmelo flavor. I'm not sure the gap between them is that wide, but I don't think it's as easy a call as you seem to make it appear, and certainly not in Banchero's favor when all the numbers seem to suggest otherwise.