I am surprised no one has responded to this, so here's my 2-cents.

It is really difficult to come up with a meaningful FOM to compare different PA technologies because device performance is taken to the extreme and PA application requirements (gain, linearity, efficiency, cost, reliability, packaging, matching, supply voltage, etc.) vary wildly.

In general, a good PA technology for power efficiency will permit very high currents with a very small voltage drop at the output (near short) and very high open circuit voltages (no or little current at high supply). Of course the device has to switch back and forth between high current and high supply at the highest frequency of interest, so ft for the device size necessary to deliver the required output power is useful to know. Next might come passives (caps/inductors/tlines), but that depends a lot on whether external matching is going to be used or not.

Output power and linearity swept over power might be some indication of the technology performance, but these tend to be optimized for specific applications using load pull characterization.

Reliability and ruggedness are also important. PA for mobile applications have to tolerate fairly extreme changes in VSWR.

It would be interesting to hear what others have to say about picking PA technologies (in general).