Doesn't matter which is better. I use AIM because all of my friends have AIM/AOL. In fact, I can't think of anyone period in the United States who doesn't use AIM. It seems like every apartment, house, dorm, I've been to ever had AIM running.
Maybe you should check the poll results, I'm going with MSN because EVERYONE uses it and it's much easier on the eyes, which I think is important any time you are looking at a screen for long periods of time.