There was a widely reported rumor that the 360 version of Assassin’s Creed would feature better AI than the PS3 version. Ubisoft has officially denied this:
While the method for distributing AI load is different on each platform, the AI code itself is the same. Players will experience the exact same crowd results on PS3 and [...]
"Assassin?s Creed AI Rumor Officially Debunked" - 08.10.06