The 7100t should be usable as a modem for a laptop, see: HOWTO: Use BlackBerry As Modem For Laptop
Did you have problems setting up your BlackBerry as a modem?
What kind of software do you need on a phone? There is more software on Palm, as long as you don't need to multitask much (something a BlackBerry can do better than a Treo). The Treo has a much faster CPU than the 7100, for example. One of the biggest problems is finding the right software. For example, Google Maps is now available on BlackBerry. And there are many good quality chat software programs (There is chat software vastly superior to the one built into the 7100). Plus some models of BlackBerry are much faster than the 7100, which can mean it is more pratical to run other more intensive software.
The Treo650 may be better, but it may not be. You may want to monitor this: Treo Users Converting to BlackBerry 8700
Since you are eligible for a free Treo (hopefully no contract), you may want to try it out and see if it works better; but you might end up hurrying back to BlackBerryLand -- depending on what you need more.
Another note. Transitioning from a 7100 keyboard to a Treo or 8700 may be a temporarily frustrating experience, though easier to transition to the 8700 due to its larger keyboard.