I asked the same question on the SPA Bulletin Board and Bruce Underwood told me that 4 strokes were favored because of reliabity and torque (prop selection). With respect to the OS, he felt the OS broke in easier, was less fussy to adjust (and keep adjusted), and requred much less maintenance. I'm building a Sun Fli III right now, and I'm putting an OS 91FS in it. I'm hopeful of attending a few SPA events this year. Regards,..WarpedWing.