Depends on the line.
With a great oline I take Alexander - he had such great vision,instincts and moves and significantly better top end speed than Marshawn.
With a great oline, obviously Lynch would be great but not-Alexander great.
Where Lynch gets the adavantage is if the the oline isn't special. Marshawn can run the ball not matter what the blocking is like becasue he is so hard to tackle and plays with so much passion.
If I have to draft one not knowing what my line is like I am taking Lynch because he is instant running game and brings a toughness that SA never did. I think both guys are ideally suited to the versions of the Seahawks teams they play on. Marshawn would never look as bas as SA did when the line collapsed though.
Give me Marshawn, I much prefer the identity that his style provides of Alexander's - and I am big fan of SA but his running was condusive to looking great behind a great oline.
Lynch is all heart, SA was the natural.
