It's based on watching both QBs. Lamar shocked the league with his athleticism. It was clear that by the end of the season, teams learned how to defend against him (take away the middle of the field; don't let him run). Buffalo should have beaten the Ravens and did a great job containing Jackson. Two years in a row, we've seen what playoff defenses do to him.
I think he's been exposed and the Ravens' window was last year ... now shut.
I think Allen is much more versatile, much smarter, will end up being more durable (Jackson won't last long running the ball that much).
Jackson had a PHENOMENAL season last year and my hat's off to the young man. But he will never come close to duplicating it.
Josh's ceiling is simply higher, IMO.