The difference in cost is about $100. The difference in features is huge. Examples: the 5x versus 3.2 camera and the "extra" slide out keyboard.
Speed at every corner is a major difference. From the faster processor to the multitasking, the difference is significant.
"I want an Android phone, but if I don't think all of the bells and whistles on the Droid are worth it, I may go with the Eris, which is really nice."
- http://googlewatch.eweek.com/content/hello_android/google_advertises_motorola_droid_on_its_home_page.html (view on Google Sidewiki)