Both great choices. How well they run is more about how well you can tune and engine. Another thing is the OS may work out better if you require a bit of extra weight in the nose.
The trainer I have on hand at the moment has an OS .91 four stroke in it. My other engine choice was a SK .80, both good engines but I enjoy a four stroke more then a two stroke. It's a choice thing. Both your choices are good and will work just fine.