I've had both and I can honestly say that Symbian is the most advanced OS. However, the Treo 600 is a huge leap compared to the Treo 300. I'm getting acustomed to my Treo 600, but it is quite stable. If it crashes, it's mostly because of an app that it's in trial mode (guess that's part of the catch for you to buy it!).

The Treo is an e-mail machine, but browsing to the internet is not so enjoyable. The Opera browser for the P800 is GREAT, everyone should have it. It's very stable and fast. Too bad it's not available for Palm... I'm still in the process of trying browsers so watch this space...