AC2 is a better game, but I like AC1 more. In the first one, there was actually some feel to being an assassin. The minigames were repetative, sure, but it was really satisfying to get a map of the guard locations and makes plans accordingly. Also, you could actually make a few stealth assassinations if took the time for them.
AC2 throws all that out the window. All the assassinations are "Here's the guy, go kill him now." On top of that, everytime you try and sneak up on a guy, a cutscene start playing and the dude start running away. Not to mention that the notoriety system was broken, and could easily swordfight your way out of anything, even if you sucked. AC2 isn't about an assassin. Its about a guy with a sword that can free-run when he needs to get from point A to point B.
Also, Ezio is a doosh compared to Altair. But I suppose its fitting that the difference in character reflects the difference in gameplay.