I keep forgetting you can throw on a 3 blade prop. The 100 turns a prop 1" larger in diameter than the 91, and I didn't think the 100 would have proper clearance for the blades at about 15". The 100 is about only $10 or so more than the 91, but you will have to learn very early on what throttle management is. That thing will be a rocket, if you ever punch it on take off , you will flip it over on it's side.