AIM is decent for having a casual conversation. I use it mainly for real life friends.
MSN is more organized and all around easier to use, therefore I prefer it for pretty much everything. If more of my real life friends used MSN, I wouldn't even bother with AIM.
-*It's more organized: It's pretty easy to arrange and find friends on your lists. Not a real time consumer.
-*It's pretty popular: If it's not the absolute most popular instant messaging software in the world, it's in the top 5, guarenteed. That allows more communication towards a larger and diverse variety.
-*It's constantly updating: Yeah... It basically says all but microsoft updates the software pretty frequently (in comparison to some competitors) and usually, those updates truly enhance the experience.
That's all the reasons I can think of right now but I'll edit as I remember