chadwickz71,

All around performance!! I have a Saito 100 and the OS 1.08. The OS is on my Ultimate. I thought about putting the 100 on there but I do not want any less power. If I was forced to make your choice I'd go with the 91FX just for the power but in the back of my mind I'd be wanting to put a Saito 150 on it. I think if you use the 100 it will fly and fly fine just different from the OS 91. I see my two strokes flying faster and my four strokes having a constant but slower speed. There are lots of people with these planes, maybe they will chime in with their thoughts.