Hi, name's Ken(Starfighter). My vote is neither of those. Mine flew AWESOME w/a ST .75 w/Pitts muffler. ALL the power you'll need, plus lighter than either of the others you are contemplating. A friend has it now, was gonna put the .91FX on it until his first flight w/the ST .75, and he was convinced that the ST was MORE than enough to get the plane to perform. If I had to choose between the engines you list, my vote goes to the .91FX w/Pitts muffler(Slimline will fit the best. Get the 70deg. exhaust outlets version. The Bisson muffler is too massive in design to fit well in this application.) That's my 2cents worth. Good luck!