The BlackBerry device is designed to be an email/messaging device first, phone second. It has a email push technology that ensures you get email messages anytime. It has some basic PIM functions for you to keep contacts, calendars and such.
The Treo 600 is a PDA plus phone device and have recently added the Good Technology push email bit so that it can get emails pushed to the mobile devices.
From our hands-on experience, the BlackBerry is more integrated for email purposes.