Between he and Rosen, Allen is more likely to succeed early. The idea that Rosen is more NFL ready is misleading. Yeah, he has better mechanics and is better at reading defenses, but if you ask him to play like a vet NFL QB right away, he's going to get killed. Rookies that have the most success are treated with kid gloves. They are asked to make one read, roll out, dump off, etc. Everything is very deliberate. Allen, due to his athleticism, is much more suited for this.
Examples of rookies succeeding right away are Rothlisberger, Wilson, Watson, and Prescott. Look for the Bills to run this type of simplified offense.
While Rosen, IMO, has more potential to be great, Allen is more likely to succeed early.