Mallett dropped from the first round to the third largely because of character/maturity issues. If he had been drafted in the first, we would have seen by now what he was or wasn't because he would have gotten the opportunity dictated by draft position. If Mallett hasn't developed by this point, chances are he won't turn into the next great QB. We saw some good things, but it's not like he blew us away or anything. Still, if the coaches feel he's where he needs to be at this point in his career, I'd rather have him at the helm than a rookie, no doubt. Nonetheless, you draft Hundley if you feel like he has the potential to bloom into the guy you want under tutelage.
As for specific differences at the same points in their careers, I think Mallett has a bigger arm, but beyond the point where it actually matters. Hundley doesn't have the same issues with touch passes. Hundley has a better feel in the pocket, in terms of ability to feel the pressure and slide. To my eye, Hundley has a quicker release, which is nice. They both have accuracy issues, but I tie that to footwork, which is completely correctable with pro coaching as long as the player can be coached. Both are very intelligent football players. Hundley is obviously far more athletic than Mallett, but more importantly, he has demonstrated the mentality to extend plays behind the LOS instead of beyond. Hundley has demonstrated the ability to feel the rush and slide in the pocket. There was a stretch during the season when I felt like he was getting happy feet, but the last couple games of the season he really cleaned that up. That athleticism coupled with the mentality to pass first is a key difference.
So there's some similarity, to be sure, but Hundley scores higher in having a quicker release, better athleticism, the mentality to use it in an NFL style, and doesn't overly rely on his arm strength. Part of what got/gets Mallett into trouble is the belief that his arm is so strong he can do anything. That results in some bad decisions.