A shoulder belt , with a retractor in the seat back, requires the seat back be stronger and the hinges have better locks to retain their adjustment in the event of a crash.
I've had P/Us with both types and found the belts mounted to the truck body more comfortable ; maybe because of my height; the seat back type was always tight over my shoulder.
Safety wise , because of the M/H's swivel seat, I'd trust the wall mount to keep me and the seat, restrained in the event of accident.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.