They both use the same chassis with similar external connections and features. However, the difference lies in the electronics. For the Amp electronics, the Desktop Amp uses the Home module while the Ultra Desktop Amp uses the Max module. You can read the details here. For the DAC electronics, the Desktop Amp uses the Home DAC module while the Ultra Desktop uses the Max DAC. You can read more about that here.
I had a Micro Amp so I know that the home electronics sound great. The general sonic character is the same but with the Max electronics there's improved detail and dynamics. The HeadRoom amps pair very well with the HD650's.
Oh yeah, why aren't Headroom's amps mentioned/used around this forum?? Seems like a great product.