Hi Andy!
Well, the P-90 is a development of the old trusted P-80. A great idea, but a different story compared to the P-100. The P-100 is a great engine, we have used ours in a Turbinator, Reaction 54, C-ARF Mig-15 and Tomahawk Viperjet so far. It runs flawless and has plenty of power for the size and weight. I would highly recommend that engine. The P-90 is able to produce similar thrust at lower rpm because of it's bigger size. It has not only an overall bigger diameter, but a bigger compressor, and a bigger turbine-wheel, so putting the same amount of air through the engine is possible at lower rpm. Some sort of phyisc I guess
. The P-90 is developed to push the old P-80 to a higher level with onboard everything. Just plug the turbine to your fuel-system and to the ecu and your done.
I would go for the P-100 again. A great engine with sophisticated accessories.
BR,
David