MSN will win this fight, because of a relatively new program called MSN Plus, which can be found at the URL http://www.msgplus.net/
This useful program adds a new menu to the MSN window itself, as well as all chat windows. It allows chat logging (although I believe this is an option in MSN version 6 now), it allows custom away messages with auto messages, there are a series of sound effects that are activated with /s(command), and for IRC users commands such as /whois, /me, and /away can be used.
Another IRC compatible feature is the format tags used to give different sections of a message different colours etc. I found that if one copy and pastes a coloured message from IRC it will show up the same colours in MSN Plus.
I suppose what I'm getting at is this; MSN will win the IM battle, because people love to annoy Microsoft by making programs such as MSN Plus, and programs made by this type of people are usually better than any made by a programmer.