The ST 90 is a shaker. I've had two and they both vibrated badly enough to shake the airplane apart. Your airframe will last much longer with the Saito 100. Most people prefer the 4 strokes for aerobatics anyway because they accelerate faster from low throttle settings and spin a nice wide prop. An APC 16 x 4W on that 100 will yank that Edge out of a hover.

Jim