Well, I don't have an OS 91 yet. I am contemplating which engine to get. I have 5 choices, going from top to bottom in both preference:

YS 91 $275, best power and throttle response
Saito 100 $279, lightest
OS 91 Surpass $229, easiest to handle (quietest?)
OS 91 FX $200, good all around
Super Tigre G90 $150, cheapest

So depending on what is needed for the plane, I will optimize the dollars. I think I will get the YS if I can find one locally.