Theres been a few comparisons for graphics, and the general consensus is that Xbox usually beats PS3(the differences are minute, its usually just shading). The controls, on the other hand, are slightly better on PS3. I played Assassin's Creed 2 on PS3, and Brotherhood and Revalations on Xbox. I prefer the xbox version, but not by much.
That being said, PC > Console version. I'll be getting Assassin's Creed 3 on PC when it comes out(whenever that may be, they keep delaying it)