Assassin’s Creed developer Ubisoft says that while the Xbox360 and PS3 versions are “nearly identical”, the Xbox360 game will have a better AI system. According to an IGN article, Producer Jade Raymond says,
“The hardware also allows for improved threading, which will improve even further the crowd AI,”
The article only mentions “crowd AI”, and not enemy AI, so the difference is probably insignificant. However, it suggests that the Xbox360 hardware is easier to work with, as the chip architecture is simpler to that of the PS3.