I have a 100 in mine. I would highly recommend going with the 91 if you have it. If not get the 100. You will not need to run 30% with the 91 or 100. I run 15%, and i could easily run 10% if i wanted to. I had the .72 on mine and with a 13-4W prop and 15% it had no reserve left to pull out of a hover. You will hear a lot of opinions on this topic. I have had both engines in the same plane and bigger is better. It is still just as stable with the 100 in it as it was with the .72 except now i have plenty of power to get me out of trouble if need be. Hovering now is below half throttle spinning a 15-4w apc prop.
Al