GHM has a lower height over bore, but the APC is a shorter platform (as the receiver doesn't stick out behind the grip as much).
Pro model of the APC gets the advantage of a non reciprocating charging handle if that matters to you.
The supposed big advantage to the GHM was that the hydraulic buffer was to be included into the carrier so that the stocks weren't so expensive, but instead of pricing the collapsible stock for it more reasonably because it doesn't have its own hydraulic buffer, they just priced it the same and pocketed the difference.