I waffled between the Saito 120 and the YS91. I went with the Saito. It seems to be just fine for balance with some careful planning. I'm probably sacrificing 1/2 pound to the YS. On thing for sure though, the 120 is a tighter fit in the cowling.

I personally see no reason to go bigger than 120. Even though there will be no difference in weight, the motors cost more, shake more, and have a greater potential of damaging the airframe when the throttle is abused.