Kelly was great and he was tough as nails, but he had no where near the skills that Allen has. What Kelly had was the Option of NFL MVP and HOFer Thurman Thomas at his side. Kelly was the Leader of the Bills, but Thurman Thomas was the piece of the puzzle that made it possible. Allen is like Thurman, he is so dangerous as a player with his multiple skill sets. Kelly was a game manager that called the plays at the line, Allen is a game changer that can make plays Kelly physically could not.
Kelly was a star at Miami and was drafted as an expected starting QB. Kelly had a serious attitude of being superior when drafted and went after the money in Houston before that league folded. Allen was a question mark and seemed quite humble coming into the league. Allen had to earn everything before and after the draft. Allens attitude and potential got him here.
Kelly is a different person now after suffering some life tragedies, he grew as a person later in life. Josh is years ahead of him in that regard.
Both are great, but in their own way.