As a USC alum I've got to identify Leinart. But Leinart with 5% more arm strength makes this a non-question.
Leinart has far more variety of pace and loft than Beck. He can drop it off a roof top when necessary.
And the problem with Beck's low delivery is not merely batted down passes. That's the cheap and convenient reference point. It's all about angle of attack, similar to golf. When you have the arm and elbow going forward in ideal angle and proximity to your ear it's simple to adjust loft and make almost any throw. But now try that when the ball is low, out toward your side. At that point the wrist is more flat, the palm more open, and the successful permutations much more limited. It's why you see line drive throwers among QBs with low deliveries. Not much different than flat golf swings like Trevino's producing low shots, and not nearly the variety of a more upright contemporary like Nicklaus.
The only thing I noted about Beck last year is the release has to be repaired. The other stuff will work its way out, like ball security. He's actually decent in short passes but the arm dips too low on deeper throws.
I don't want this to sound like a knock on Leinart b/c I really liked him coming out but Beck has just as much touch. He also tends to throw from many release angles depending on the type of pass. Check out this video from the preseason.
http://s224.photobucket.com/albums/dd1/bytesandbits/?action=view¤t=2007JBExhibitionPerf.flv
The last two passes in particular come out much higher. They're not perfectly over the top but they are far from flat trajectories.
I don't think people realize how good of a passer Beck is. I'm not a BYU fan but I live out west and being too much of a football junkie I watched several of Beck's games. The accuracy this kid has jumped off the screen at me more so than any QBs since Rivers and before that Aikman. To me this is the most important characteristic but he has everything else as a passer as well. His release is very fast (not Marino, but as fast as just about anybody else). He has incredible touch. So much so that people thought he lacked arm strength. This was also incorrect as we've all heard (or seen) his 70 yd pass (actually 65), the pass from his knees at the 50 through the uprights and radar gun timing at the combine. Also his timing and anticipation are incredible as Jaws demonstrated during the draft.
I think people get too reliant on media rankings and think those are representative of what QBs need to succeed. Those rankings are often based on useless criteria. I believe that if Beck were 2" taller and 2 years younger he would have been rated as a 1st round franchise QB by everybody. I however, don't find those two characteristics to be much of a detriment to his success. If he had gone to a bigger name school I think he would have been a #1 overall pick. (BYU played against 5 top 25 ranked pass defenses in 2006 but people will persist with their perceptions). I would put Beck's passing ability up against any QB in the league. He wouldn't be the best at every nuance but he'd be among the best. On top of that he has the mental makeup and work ethic to utilize his passing prowess. If it isn't already obvious, I have a great deal of confidence that Beck will be a very good NFL QB.