Well, you can use them both together.
You cannot use MSN messenger and add your jabber contacts, but you can use Jabber IM, Trillian, Everybudy or Miranda and have all your contacts together (even from yahoo and AIM in some cases). To do this, the best thing is to take a look at BBB Interoperability Matrix, and choose the best client for your needs...
http://www.bigblueball.com/im/interoperability.asp
I think there are other ways of having Jabber and MSN contacts together by using certain Jabber servers....but as I am not a Jabber user I can´t tell you exactly what to do.
N.-
