I also have had both.
A lot of good opinions have been expressed above by people who have also had/have both.
I echo a lot of what they have said. Here's what my personal experience is, and take into effect i have only had the Hero a week, i had the Pre for 3 months.
Hero Advantages:
-Build Quality... FAR & AWAY better than the Pre. The Pre is plastic, twists, and feels cheap. The Hero feels solid, especially when you have been using the Pre for months. And the glass screen has a much better feel than the plastic screen on the Pre (i think it's plastic)
-Battery Life.... FAR & AWAY better than the Pre. I use my phone i'd say moderately. I text 30ish times a day, a few emails, moderate web browsing, sometimes music/podcasts. With an extended battery on the Pre (Seidio 1350mah), my Pre would BARELY make it through the day, sometimes it would require a charge before the day was over. And if you were ever in a low coverage area, forget it, your battery is as good as dead. The Hero, with the same usage, lasts easily throughout the day and then some.
-App Store - 10,000 vs. 150. 'nuff said.
-Widgets - i never used Widgets before, but i love them. I love having a constantly updating Facebook widget, weather, Calwidgets, etc.
-Scenes - technically with a lot of stuff on the Pre, you can just leave the card up and swipe back and forth, but the scenes are completely customizable to have anything and everything on there. The customization is just incredible, and way beyond what the Pre is capable of.
-Misc - Hero has text forwarding, video camera and LED notifications, and the Pre does not.
Pre Advantages:
-Multi-tasking: The Pre KILLS the Hero in this. Yes, you can multi-task on the Hero, but not anywhere near the ease and fluidness of the Pre. Also, with the Hero, you kind of have to moderate what stays open a la Windows Mobile. With the Pre, you know exactly what is running and what is not.
-Gesture Area: I really miss this a lot. The Gesture area, once you start to using it, was second-hand, to go back, forward and switch applications
-Keyboard: I do prefer a physical keyboard, and yes the Pre had what some say is a small keyboard, but i never had a problem plugging away on it. The Hero's virtual keyboard is really responsive and the error correction is great, but it still cannot beat a physical keyboard.
-Camera: I do not know why, but the Pre's camera (which has flash) was the best camera i have ever used on a phone, hands down.
-Notifications: They are similar, but the Pre's notification system was a lot nicer and easier to use and more intuitive.
-Pandora/Music: It stayed at the bottom of the screen for easy access if you were on another program
With all that being said, for me, right now, the Hero is a better fit for me with the apps, build quality and customization the Pre sorely lacks. Future iterations of webOS will no doubt be fierce competition, but that is in time. For now, i am sticking with the Hero.