My practice: Set proper CG with heaviest battery and use the heaviest (more powerful) battery.
Power is your friend, especially on takeoff. Better to have it and don't need it, than to need it and don't have it.
If you want lower cruise-speed, reduce throttle.
Optionally, you could reduce the prop size to reduce thrust, and then use a more optimum prop later to increase performance. But that would eliminate your reserve power for takeoff.