Both the OS and Saito are good engines. I had a Saito 100 on a plane for a while, then I removed it to put on another plane and put a new OS 91 4S in its place. The OS is heavier and is not nearly as powerful as the Saito 100. My two Saito 100s took no longer to break-in than any other four stroke that I have used. Either engine would probably work for you but you might also consider the Saito 91.
Fred B