AC:B runs like a dream, AC2 seemed to have a good framerate, but it wasn't upto par with AC:B. I'd occasionally dip to around 25-30 in high population areas for a moment or so in AC2, but I haven't ran into this problem in AC:B. The graphics options are more customizable than in AC2 as well.
It seems they optimized the game for PC's quite a bit during the delay on the PC version. They also remapped all the keys to look like it wasn't a Console port (for example you have "Space" under your sprint button, and E on your Eagle vision). Quicktime events also show which button and mouse button to press.
At the start of AC2, I missed a few conversation quicktime events simply because I fumbled around and forgot which key was which.
I was just stating this exact thing on another topic. They ported Brotherhood WAY WAY WAYYY better than how they did AC2. I'm loving the flawless gameplay, except the mouse acceleration but since it isn't an FPS, I'm fine with that.