Voted for MSN, only after a long debate though...

I like the direct connect of AIM, its very simple to share files easily and quickly. It also has a simple interface imo.

However, MSN is much cleaner. I like how you can change your display name, and the emoticons are great (especially (ci)) haha.

Anyways, theres more reasons than that. I use both just about the same, mainly cus the entire east coast uses AIM, and then I moved to the west, and everyone here uses MSN, so im kind of torn...