Both the devices are equipped with all the hardware features that one would expect from them. This includes, a high resolution capacitive multi-touch screen, 5 mega-pixel camera and so on.
Below is a quick rundown showing how the two Droid phones stand against each other:
From the above comparison chart, it is pretty clear that Motorol
a Droid is a device with a higher profile in comparison to HTC Droid Eris, both in terms of hardware and software. However there are a few other options to consider, for example, you may or may not want a physical QWERTY keyboard, or full multi-touch support like in Sense UI.