Hey Stephan,
In the end it makes no great difference and is largely just a question of convenience.
If you plan to offroad regularly (say once a week on average), then having a fixed onboard compressor is useful and is one less thing to pack and remember.
If you want to avoid making any changes to your car, or you’re happy to pack your equipment each time you go off-roading, a portable unit is a great option.
I went for a fixed installation onboard compressor in my Jeep as I take it out regularly enough to benefit from the saving of time and space taken up in the car.