Altair is much more of a cold, clinical character. Some may consider him a true assassin. However, with Ezio, we have watched him grow, he plays the much more emotional part. They are both two contrasting characters. My favorite is Ezio. He does seem more 'Human', and injects a great deal of humor into the franchise. Although we shouldn't forget that as a non-playable character in Revelations, we see how Altair has matured and grown wise - a long journey from his former self.