My nod goes to the Saito 100. I flew an OS 91 FS prior to installing the 100 and the Saito is a much better choice. We have a guy in our club that flies his with a .91FX and it's a good combo as well. Personally I like the four strokes better for this plane and the type of flying it was intended for.

On the subject of servos, if your budget allows for it, go digital. The Hitec 5645/5625 are perfect additions to this plane. I started out with 605BB's in mine and simply didn't have the controllability I wanted. Replaced the rudder sero with a 5645 and the elevatos with 5625's and now it's dead locked in in a hover, made a HUGE difference in the controllability of the airplane. Ailerons can be analog, I have 545BB's on mine and it flies fine with them. Replace all of the control rods with 4-40, it's a must.

Go digital, you won't be sorry dou did!