I´ve been doing some research in phones because I need a new one aswell.
I have no personal experience, but specification wise the HTC is best.
The HTC and nexus one both run on android, which is becoming a very nice mobile operating system. It even supports flash in web browser in their latest version (available for both the nexus and HTC).
There also is a marketplace with very nice apps, there aren't as much as on the iphone tough. I think that the HTC is better in basicly every aspect than the nexus one, so the real battle is between the iphone and the HTC.
I would take a look at these reviews, they cover pretty much everything:
http://www.engadget.com/2010/04/01/htc-desire-review/