Since there is quite a difference in weight between the two engines, and you've got the airplane back together, why not figure out how much weight it will take to balance out the airplane and let that help you decide which engine to use. Since they both are excellent engines, and you don't have a preference, let the airplane decide.

Put whatever weight on the motor mounts that it takes to balance the airplane. Then see which engine is closest to that weight.

They're bot excellent engines. The OS is about 2oz heavier than the 100FA and is about $30 cheaper.