In girl mode its yahoo, like Karren said it links my 360 page and my email and notifys me instantly of any emails or messages. Can chat to people on MSN through yahoo too.
In boy mode I use MSN mainly for family, I have a deaf sister and its real handy for her.
I run them both at the same time, I instantly know which mode when it pops up (yahoo girl mode) (MSN boy mode)
Life is so complicated lol