I had both during about two weeks of overlapping 30-day evaluation periods and ended up keeping the Hero. My thoughts:
1. At full brightness the Hero has the brighter screen (see below). In addition, photographs displayed on the Hero's screen had higher and more-pleasing contrast.
2. The screen sizes and resolution are clearly different but the higher resolution of the Droid made no difference in readability to me and, as can be seen, there's little difference in screen real estate. Only a few more lines of Google News show in the screen above.
3. The Droid seemed faster at doing things when connection speed wasn't an issue.
4. The Droid weighs exactly one-third more than the Hero - 170 grams vs. 126 - and because of its square corners and flat profile it really holds like a brick in your hand. When I'd leave the house I'd find myself grabbing the Hero because the Droid was just to big, pointy, and heavy. I didn't want it in my hand or my pocket.
5. The Droid's hardware keyboard, though drawing negative comments, was easier for me to type on than the Hero's landscape s/w keyboard. Since going full-time with the Hero I've found it progressively easier to use its keyboard.
6. I used Sprint Nav and Google Nav simultaneously on a 400-mile trip. Google was quirky and sometimes urged me to take pointless detours from the direct route (something that will doubtless be improved) but it was much better than Sprint Nav. It was more customizable and gave a much better display of the surrounding roads and countryside regardless of the zoom level or orientation. But see #8 below.
7. I liked Droid's email app with its combined mailboxes better. On the other hand I missed pinch zooming on the Droid and didn't like tapping or selecting + and - to zoom.
8. Google Nav and better email will accompany Android 2.0 when it reaches the Hero, so those advantages for Droid are only temporary. Its massiveness is permanent.
For my particular situation it was going to cost me about the same per month for either phone. For me the Hero seemed to be the right choice.