RE: OS or Saito for ST50
Advertised HP is a worthless number for 3D. The only time that matters is at full throttle, full speed.
Most of 3D happens at half or less throttle. What matters is thrust, and how quickly an engine can make it.
The OS91 is overweight for it's power (as it pertains to 3D).
Get a Saito 100 at 21.2 oz (with muffler). It will be lighter and more powerful.
Remember that airframe needs to be kept light. Ignoring this will give you very poor 3D.