But ... You can use a .70 on a .40 size plane. The 4S will swing a 14X6 propeller about 9500 RPMs where as the .40 2S will turn an 11X4 about 12,000 rpms (comparitively speaking). On lighter planes like profiles and 3D, the extra weight of the 4 stroke is offset by it's ability to swing a large prop. On warbirds, people like the 4 strokes because of the sound (sounds more like a lawnmower as opposed to a weed eater). I think it's just a matter of personal taste. Power wise, the same size 2 stroke is more powerfull, but they weigh more.