I had a Blackberry 7510 and 7520 for Nextel, and now have a Treo 650 for Cingular. Blackberrys only advantage over the Treo's are that they have direct email push, where as on the Treo you can only schedule to check your email or you need to purchase 3rd party software.

Blackberrys have trackwheels, the Treo's have a navagation pad and stylus. I like both, and I'd say it's a tie.

As far as the OS, Palm destroys the Blackberry OS.

Blackberry is only an email device and a phone. The Treo is a true PDA\phone. If email is important to you, go with a BB. Otherwise, use the Treo