I had the HTC Hero (Sprint's version of the Eris) for a week and the Droid for a week, with two days where I had both.
They're both great phones. The HTC hero had some critical bug issues which had to do with the battery life. There were workarounds but I was always paranoid I didn't do the workaround and that the battery would drain right away. Besides that I loved the compact Qwerty keyboard when using the onscreen keyboard in portrait mode. It was so quick to type messages in that mode and the predictive software was excellent. The droid lacks that function and the predictive software is like crap compared to the HTC's.
I also liked the camera better with the Hero. Not nearly as much noise in the shadows and in low light situations as the Droid. There should be a partial fix for the Droid, but I do believe it's a good chance it's the hardware. Noise in photos when taken in the same conditions by two cameras is usually due to an inferior sensor... Though I guess it could be that the camera software is bumping up the ISO unnecessarily.
HTC has a lot going for it. The SENSE UI is not just aesthetics. It provided more option. For example, the camera software allowed you to pick the iso, auto focus on a specific area by pointing to it, etc, etc. It's just a refined and well thought out UI. I understand that vanilla Android is supposed to be plain and you're supposed to add to it. Hopefully, the support for things like picking ISO and the focus on where you point will come. It's just not there yet.
I ended up keeping the Droid because of the snappiness of it and the physical keyboard. It makes a difference in multi tasking and in web browsing. The processor is able to render the pages faster. I tested them side by side so I know there is a difference. Also, I didn't want to count on HTC to provide me with the Android OS updates. They can choose at anytime to stop updating and even wehn they do update it, it will take a bit longer because they have to integrate SENSE.
However, in the end, I would have been happy with both.