First of all, I have watched alot of Alex Smith. He is very athletic and appears to possess great decision making skils. However, he needs to add some weight to improve his arm. He also needs to work on some of the finer parts of the quarterback position. The bottom line is that the aspects of his game in which he needs work are all correctable. He will need to hold a clip board for 1 to 2 years before he is ready to really make an impact, however, when he is ready we may have a real gem on our hands.
Alex is 20 years old right now. He will turn 21 in May. That means that after two years of holding a clipboard, which is often the best way to develope a quarterback (McNair, McNabb, Pennington, T. Green, Bulger, Hasselbeck, etc, etc) he will be 23 years of age. As a comparison, at that same time Ronnie Brown will be a 26 year old tailback.
If Saban wants an almost sure thing impact player who can make a difference right away, that player is Ronnie Brown.
However, if he wants someone who may not play for a year or two but might be a staple for the next 13 years or so after that, the decision is simple.
In the words of Nick Saban, Alex Smith may have the "best value to the organization".