Both Alex Smith and Matt Leinart have struggled for their franchises since being drafted in '05 and '06 respectively. Both have shown promise at times but haven't been able to deliver on a consistent basis. I think the two are pretty close but if I had to choose one I would go with Alex Smith simply because of the form he displayed last season.
He showed that he could lead the franchise well and compiled a respectable 81.5 passer rating through 10 starts. He will only improve with continuity and the weapons he has surrounding him including: Pro Bowlers Vernon Davis and Frank Gore as well as emerging Wideout Michael Crabtree.
After showing some promise in his rookie season, Matt Leinart has had a poor last 3 seasons for the Cardinals. He has started only 6 games and thrown 3 TD's to 8 Ints. He will also have an improved supporting cast around him.
The battle between these two QB's is so important that it will probably decide the division. Now onto the Running Backs.....