Physically Palmer was the superior athlete, but Leinart is above average in most regards. Whereas Palmer had an A+ arm, Leinart has a B+ arm but has arguably shown better touch on the ball and a more proficient ability to go through his progressions. Leinart was given more responsibility in the huddle, which is as much a byproduct of his starting for 3 years to Palmer having one season under Chow in that system.
Leinart had the more comprehensive and successful body of work, whereas Palmer was the much more highly regarded prospect coming into college. Palmer, as most know, was considered a let down for much of his early college career but put it all together under Chow/Carroll.
Both have A+ footwork, throwing motions and release points. Leinart probably has a slightly better pocket presence, but again that's as much a byproduct of experience as anything else. [i'm referring to pocket presence coming out of college to be clear].
I don't understand this idea that Palmer is more intense or that Leinart lacks passion for the game. One could argue that someone willing to return to college to play the game versus becoming a millionaire indicates a true love for the game itself versus the business of it. Furthermore, the guy has the best winning percentage in D1 history IIRC; and he wasn't simply a game manager, he was a prolific player who won a Heisman and could've easily won a second.
In terms of entering the league, Leinart is considered more of a "sure thing" but his B+ measurables [versus Palmer's A+] will probably have a lot of people suggesting Leinart has a lower ceiling too. I simply don't believe that because from an athleticism standpoint he's on par, or better than the Mannings, Brady, Hasselbeck and Delhomme; who are among the standard bearers at the position currently.