I have and use both systems. The pneumatic works well for me, however I have trouble getting them to slow down. They are a spring air system. If I restrict the air, the movement is very choppy and not smooth. You do need more equipment to run this system and don't forget the pump at home.

The mechanicals are simpler. I would go for mechanical on a P51, the gear retract inwards and there would be a straight run of pushrod to the servo in the centre. They are easy to set up. Measure the amount of travel that the retract needs, then use a blank servo wheel and drill two holes that distance apart, hook up the pushrods and your done. The JR 791 is the best retract servo I have used, it will break or bend something before it ever stalls. I use ball links on the servo wheel, they pop off if the gear hangs up or gets jammed.