I have both a Droid and a Nexus One. I have both of them set up for VoIP calls, and both are set up the same way. Here's how I set them up:
How To Set Up VoIP on the Nexus One Google Phone using Sipdroid for Free Calls | Dave's Tech Shop
How To Set Up VoIP on the Motorola Droid on Verizon Wireless using Sipdroid for Free Calls | Dave's Tech Shop
The call quality on the Nexus One is much worse than the Droid. I did repeated tests with both. And I used WiFi, so it can't be a difference between VZW and T-Mobile.
Anyone else tried VoIP on the Nexus One yet? I wonder if I have a defective phone or if this is a bigger issue.
EDIT - The VoIP call quality problems are worse when using a headset on the Nexus One! VoIP quality on the Nexus One is marginal to poor without a headset, but with a headset it is unacceptable. I have tested four different headsets now. I have also tested those headsets on the Droid, and they are good. So the problem is the Nexus One, not any particular headset. My first thought was that I had a bad headset jack. However, after more trouble shooting I tend to think its not as simple as that. Music sounds fine through the headset. It is only VoIP sound that sucks when using a headset. The problem is so bad that phone calls using a headset are not workable.
Summary of Nexus One and VoIP:
* VoIP sound quality is marginal on WiFi with no headset.
* Music sound is good with headset on Nexus One.
* Headsets I'm testing with are good with VoIP and music on Droid.
* VoIP on Nexus One with a headset has poor sound quality.