Blackberry's have all those extra costs for the e-mail service and stuff. Get the Treo. People complain about the Palm OS, but I don't have a problem with it. Palm makes some Windows based Treo's as well.
I have the new Palm Centro and love love love it! When people see all of my phones' features they always want to steal it from me, lol.
In the end it comes down to ease of use and how all the features apply to your life. There are blogs for the Treo and Blackberry that compare the two. After doing a lot of research, I chose the Palm.
Hope this helps. Feel free to pm me if you want more info.