Hard to tell on the speed. Only way to know for sure is to try both. Speed comes down to a lot more than "Ground rpm" x "pitch".

The engine will unload differently in the air with each prop, and the plane will fly differently with each prop.

As long as the needle on the 8.5x7.5 will allow you to back it off cleanly at least 800 rpm before launch, then you should be good to go there.

But if the engine is new, you may want to run the lighter 8.5x7 prop anyway for a while. RPM will not hurt the engine... will only make it run better.