Mlive has a comparison between the two Droids:

Design: HTC Droid Eris
(But iPhone beats both Droids)
• Without a keyboard and a smaller screen, the HTC Droid Eris is much more pocketable and sleeker than the Motorola Droid. (The Eris also has a trackball.)

• Despite its name, the Eris doesn’t have the annoying “DROOOOID” sound effect and “I’m a robot from the future who is going to kill you” startup graphics that Motorola’s phone has.

• The Droid Eris speakerphone is loud, but not quite as loud as Motorola Droid.

• Like the Motorola Droid, the Droid Eris has a 3.5mm headphone jack and WiFi.

• Both Droids have a capacitive touchscreen and on-screen keyboard like the iPhone. But unlike Motorola’s Droid, the Eris supports multitouch pinching gestures for zooming in on web pages and in other applications.

• Both Droids have a 5 megapixel camera that is meh, but the Motorola Droid records video at more than double the resolution than Droid Eris (352 x 288 pixels).

• Unlike the the Motorola Droid, there is no physical button on the Eris to activate the camera.

• I had trouble with the Droid Eris screen unlocking while in my pocket and calling random people, but a software update has made the screen less sensitive while locked.

User interface: Advantage Motorola Droid
(Both Droids beat iPhone)
While both Droids handle multitasking well, the Motorola Droid runs Android 2.0 while the Eris is still running Android 1.5.

HTC chose the older operating system so it could use its Sense user interface for Android, which offers some special eye-candy widgets for such things as Facebook and Twitter. I found these widgets to be subpar to other application options on the phone, and I really missed the unified e-mail and Google navigation voice commands featured in Android 2.0.

Overall
The Droid Eris packed more of a punch than I expected. Like the Motorola Droid, I felt comfortable leaving the house without my iPhone.

While I was disappointed that the Eris didn’t have Android 2.0, rumor has it HTC does plan to push an upgrade to the phone over Verizon’s network.

While I prefer the extras the Motorola Droid offers, check out the Eris if a smaller form factor is one of your main requirements.

Read the rest of the review on Mlive