Well, think about it this way
Web:
- Bigger screen = better browsing experience. The Droid's screen is about 4x as big as the screen on my last smartphone (Helio Ocean) and while both could browse the web, the Droid's bigger screen actually makes viewing full websites realistic.
- Typing in URLs is always better with a physical keyboard, but bookmarks can eliminate the need for most URL typing.
Email:
- Reading anything long (or HTML-formatted email) is nicer on a bigger screen, but short/simple email is basically the same regardless.
- Again, having a physical keyboard is nicer for typing out decent-length e-mails.
Camera/Video:
- Video quality on the Droid beats the Eris, but unless you're comparing side-to-side it probably won't bother an Eris user too much.
- Apparently there's some glitch that doesn't allow Youtube to work on the Eris, but I imagine that will be fixed at some point.
- The Droid has some of the best camera hardware available in a smartphone (not THE best, but it's pretty high up there) and depending on lighting conditions you can get some really impressive photos & videos. It's only shortcoming is the software, which can always be improved.
GPS:
- As far as I know, it's all the same. And if I'm wrong, the difference isn't noticable enough for a non-techgeek to notice.
Facebook:
- You've got access to the same apps as any other Android device
- Skip the apps altogether and visit the touch.facebook.com site through the browser. It's better than the apps.
So, if you're limiting yourself to Verizon only, your options are:
1) Droid. Best functionality, questionable form, perfect fit.
2) Devour. Questionable functionality, quirky form, decent fit.
3) Eris. Iffy functionality, nicer form, good fit.
BTW, Do note all are good devices.
My recommendation based on the above: Eris. SenseUI is a little more user-friendly than stock Android, MotoBlur seems to be teenager-oriented, and the combination of simplicity and style is a winner. The Moto Droid would be the close 2nd, due to being a far superior piece of hardware.