If you want the best possible instant messaging, purchase a third party client and install it on the 7100. It will do a direct TCP/IP connection instead. My current favourite is WebMessenger from www.webmessenger.com
.... It works very well on my Rogers 7280.
It is not possible right now to use the 7100 as a modem right now yet. The capability is there in the hardware but not in software. As you may have already read, RIM is beta testing a modem feature for future deployment on certain models, but there is no release date -- it can be easily be next year. RIM has a history of not enabling features for a long time, so you may want to be aware of that. And who knows, if T-Mobile wants to offer the modem feature -- as that can add heavy usage to unlimited data plans.
The Treo works as an external modem using the PDANet program. That's something you can use today. Treo does not have Bluetooth without a Bluetooth card though. (Unless you want to wait for the Treo 650)
Summarization (for today)
Multimedia - Treo wins
Email - BlackBerry wins
External modem - Treo wins
Instant Messaging - BlackBerry wins
Now you can make an informed decision based on preferences! :D You may need to try both devices out to see which one is more comfortable, though, if you're still divided.