I just don't see how you can not feel like Allen and Mayfield have much more severe question marks than the other two. Accuracy and size are two huge indicators of success and each of those guys have major flaws there.
Just look at the history. In the past ten years you can really only find one example where the guys that went past #1 was better than the guy that went first (Wentz), and it's still up in the air.
http://www.nfl.com/draft/history/fulldraft?type=position
There's barely any superstars that weren't the first pick, either. Mariota still needs to pick it up, Wentz is injured and we'll have to see if he returns to form...
When you pick #1 overall, you can ensure that the guy you're picking fits your offensive scheme, locker room, and head coach. You'll have to cater your offense to Mayfield. You'll have to work on Allen's accuracy and level of competition. You should pick the guy you're sure about. You have nearly two months to get this right.