Jabber, Inc. has published a whitepaper analyzing XMPP/Jabber and SIP/SIMPLE as potential instant messaging standards. Must reading!
The full whitepaper [48k pdf] compares the verbosity of a SIMPLE message as compared to an XMPP/Jabber message as as well as a SIMPLE presence packet vs. a Jabber/XMPP presence packet. The Jabber packets are considerably smaller due to massive headers used for SIMPLE.
Disclaimer: I’m a Jabber kinda guy, and an open source/open standards kinda guy, so my judgement is clouded. You have been warned.