I've been using the Hero for the last 30 days, and I picked up a Eris yesterday, as I'm still undecided on which phone to go with for the next 2 years (perhaps the HTC Passion is going to be the winner). The key differences I've seen so far (putting aside the different networks & pricing):
Advantages for Eris
- Eris is slightly smaller, lighter, and thinner -- nice on the hands
- Button layout seems a little easier to use, given better spacing between keys
- Phone app is a little more intuitive, while in a call
- If you like black, certainly looks better
- No bloated crapware from the carrier
Advantages for Hero
- Stronger battery
- Superior glass (seems to resists smudges a little better, doesn't have the cross hatch touch sensor / visible dots, the graphics are still visible when turned on side while looking through rose-tinted sunglasses -- yeah, the last one was a surprise -- try for yourself)
- Double-click of physical menu button allows you to quickly wake device
- Sprint Navigation (until Android 2.0 and Maps comes out)
Other comments:
- When talking on the Eris, you can hear your voice slightly over the built-in ear speaker; I assume this is by design, to give the effect of home phones. Not sure if I like it, as it almost feels like an echo effect.
- If someone called on my Hero while it was paired to my BT headset, answering the call on my phone would result in the audio going through the normal handset mode -- I would have to turn on BT mode while in the call for the headset to pick up the audio. On the Eris while paired & connected to the BT headset, when I answer incoming calls via the phone, the audio comes through the BT headset right away as you would expect. Probably a small bug with the Hero BT.
- Some apps have yet to be visible in the Android Market via the Eris (eg. TV.com, PhoneMyPC, etc.) -- something Google will hopefully fix soon, since this seems to be the same OS as the Hero.