Bob,
The higher the voltage the faster the servo.
The higher the voltage the higher the current draw and the faster the battery drains.
Not sure the plane but since it is a 40 size I'm guessing servo speed is not a requirement.
I would use the 5.1 volt setting to get more flights in the day.

What regulator are you using?
I am thinking of going this route.

Good Luck,
KW_Counter