The major benefit of any WebOS device is its card-based User Interface. This emulates the closest to how you work in many windows/applicaitons on your desktop PC, better than any OS out there; it was so elegant, that it won the CES awards hands down in 2009, when it first debuted.
The next thing is gestures - these are simple hand/finger gestures that, once you learn them, make minimizing a card, or going back to a previous, or forward to the next view in a card, easy. Also, exiting an application is literally tossing the card away with your finger. You can't get more intuitive than that.
With the original WebOS devices, hardware build was an issue, and, because of the multitasking fashion WebOS uses, a faster processor was needed than the 600mghz CPU it had. When HP bought Palm, they made it clear that they want to build hi performance devices, and the CEO said as early as just last week, for the 3rd time, that they will not release a product that is "less than perfect".
So, with the TP specs, and the Pre specs, hardware wise, we are likely looking at very powerful devices, with smooth performance, if his promise holds true.
I'd go to the Palm page and look at the videos there for each device for more information.
The downside to WebOS right now, is, as the market sees it:
Only 8,000 applications.
I hope that helps.. and the best of luck in making your decision.