as kman said above, it doesnt matter at all what way you mount them

because its the wiring which will decide what way is up and which way is down.
personally , i would mount them in the vertical position, as this will give you more room to manovure within
the control panel itself and will allow for a tighter fit of all hardware overall , i.e - you can mount your buttons closer
to the joystick when its in the vertical position .
its up to you to decide , whatever you feel happy with at the end of the day is what you should opt for ,
but i would really let the dimensions of the control panel decide which way to mount the hardware if it comes to space restrictions
