PS is the highest quality mode. It’s the highest quality because it contains the most information/detail, hence the largest file. That’s how it works. Smaller file, less information. You cannot have it both ways. As to not being able to choose any other mode whilst in PS, can you be in fifth gear and first gear in a car at the same time?
Look at it from another perspective. Imagine you have two music sources; one a CD and another an AM radio and you had a choice in recording a song for a friend from either of those choices. Which would you choose?
The main reason for choosing a lesser quality would have been the cost and the recording time of the recording media.
Always choose the highest quality and then edit. PS or FX (Full HD) 25p.