I apologize for a delay, you can do either, the only difference would be the type of a bathroom fan used for this installation. If it’s installed directly over the shower stall, the fan must be listed as suitable for such location and additionally must be installed on a GFCI protected circuit.
This setup works perfectly in my home, it helps removing moisture quickly, prevents condensation buildup on the walls and ceiling. However, since you don’t have a door separating your shower stall area from the rest of the bathroom, installing the fan in a central location / outside of the shower base ceiling projection should work as well.
Let me now if you have more questions, just click “comment” below.