AC 2 or AC B

I need some help deciding in which assassins creed game I should get from the steam store. I sadly have a mac computer so I can only pick between Assassins Creed 2 and Assassins Creed Brotherhood...but can't deside which one to pick. If I get AC:B will I be left wondering/confused about the lore? Is the story line/gameplay better for the other? I need help!