IMO the AC games have gotten better with each installment. The second one is more similar to Brotherhood than it is to the AC1 so you don't have to worry about it being repetitive. If you enjoyed AC:B I don't see any reason why you wouldn't enjoy AC:2.
The combat progressively gets better. And that trend seems to continue with Revelations. And since combat is one of the main qualities of AC, going backwards will always result in a bit of disappointment. If I remember correctly, ACB introduced the combo
battles, where you could kill one enemy and go straight to the next with a killshot. That's not in ACII and it really made ACB awesome.
Story wise, AC2 is the better game as brotherhood adds a whole lotta nothing and the story itself doesn't really progress from AC2. However, Brotherhood does have it's merits as it introduces MP, allows for different strategies for completing missions amongst
Assassins Creed 2 is exactly the same as Brotherhood minus assassin recruits, chaining attacks, and multiplayer. Just think of Brotherhood without those things. And if it's something you want to play, there is your answer
If your an AC fan ( and have the time) AC2 is a must. You are right AC1 is kinda slow going at first. I thought it ( ac1) picked up pace after the first few missions. AC2 is a great game along with ACB.
I think AC 2 is better. It's not really what I'd call "challenging," but it is more difficult than AC:B, which was just too easy imo. In AC 2 the fat ones are actually a bit of a bother. You can't just tap x after killing a weak enemy to kill them.
Plus the world is more varied. Each new city in AC 2 looks noticably different in style, and the opening of each one to you really creates a sense of progression.