The loader type preference is something that should normally be set in the airplane configuration, so it will always be the same at all airports.
The one and only reason why you would want to specify the loader the in the parking customization, is in very rare cases, where there's not enough physical space in the parking spot for the loaders to maneuver, so one of them might required to be disabled, usually the front one. Yes, it allows to change the loader type from the one used by the airplane, instead of just letting disable/enable whatever loader is configured for the airplane, but it should be up to you to use that option to also change the loader type.
