Question about the in's Creed series?

For summer, I made a big list of games I never got around to playing but have heard are good, and the 3 in's Creed games currently out are on it. I've played through AC 1 and a little over halfway through AC 2, but the mechanics are starting to get kindof old. Towards the end of AC 2, or in Brotherhood, does it change any, or is it all the same? It's a good series and the story isn't bad, but I don't want to play the same game 3 times with the only differences being the story and UI