Gunn vs. VCO in terms of phase noise and THD
Per Leeson equation, using the same Q for two oscillators, the higher frequency
oscillator will have more phase noise, given all else is the same.

https://en.wikipedia.org/wiki/Leeson%27s_equation

Then we have the Gunn installed in a cavity, with coupling efficiencies different from direct L+C resonator connections into the VCO.