go with the four stroke, I have a .70 in mine and it is way overpowered, I had to add a little nose weight with the .70, a .91 FS might balance it out, I used the servo in the rear of the fuse for the rudder, if you put the servo up front and go with a pull-pull setup you can probably use the .70 without adding nose weight. My Cap out climbs another guys .40 Extra that has a .91 in it. I'm running a JZ 14-5, it gives me 11 lbs of thrust and lots of braking. This is one of the best planes I have flown, just wish it was bigger.