Either is dangerous. If you think about it, the longer bar would have a longer lever arm, and while it would have to travel further, it would require less energy to initiate the kickback, relative to the fulcrum, which is your left hand on the handlebar. With either bar, you're going to lose. Few are the men that can notice and overpower a solid kickback. Little jumps here and there are manageable, but best avoided, but a solid intense kickback on any saw is gonna hurt, short or long bar. A longer bar may weigh more and require more engery to move all the way back to your face, but it'll also be more akward and cumbersome to combat. Each comprimise between short and long, equals itself out to being just as dangerous as the other. I never think about how long the bar is, I just think about kickback. Period.
Jeff