Here's why, according to Charlie Rosen:
# He's much stronger than CP3 and should therefore be more resistant to serious injuries.
# Because of his size and strength, Williams can post up smaller opponents (like Paul), and can avoid being posted himself (unlike Paul).
# It should be expected that an injury or Father Time will eventually diminish Paul's amazing speed and quickness — and when speed guards lose a step, their effectiveness is greatly reduced.
# Williams is much more versatile, so much so that it's not inconceivable he could make a successful switch to the shooting-guard spot as he ages.
# Williams is a much, much better jump-shooter than CP3.
# Whereas Paul does most of his half-court scoring in conjunction with high screen/rolls, Williams benefits from weak-side screens, staggered screens, and isolations.
# Williams goes left better than Paul.
# In half-court situations, Williams' power makes him a better finisher.
# Paul is a sniper-type defender, while Williams' defense is more fundamental and less chancy.
None of this is meant to suggest that Paul is anything less than the most dangerous speed guard in the league. He's also an incredible passer, and he plays in a system that maximizes his considerable skills.
But, I'll bet your mortgage that Williams will have a better all-around career than Paul.