The bottom of the article links to the history (individual features) of other IM programs from that era as well like ICQ and Yahoo Messenger.
AIM was released in 1997, MSN in 1999. AIM was at the time the biggest ISP in the United States, so AIM was pretty uniquely marketed to us.
It was my observation that you had two main camps: Those whose home was AIM, and those whose home was MSN. And the deciding factor was probably if you used AOL as your ISP. There were people who didn’t know you could get an AIM account if you weren’t an AOL customer. Those who didn’t use AOL probably went the same way others did around the world, MSN messenger was built into Windows so it was the obvious one to use.