I think it is mainly marketing. The introduction of the Rangeman seemed to be a game changer for Casio and they have massively expanded their sensor enabled G Shock range since due to the huge success of the model.
Protreks are 100m WR compared to 200m for G Shocks. I doubt they would perform very differently really though if tested.
On shock and damage resistance and G Shocks tend to have more protection for the crystal, they both have 'floating' modules though. In the past the curved resin straps of the G Shocks would add to the shock resistance if dropped but recently Casio has been issuing more and more G Shocks with bracelets and non curved straps. So I assume Casio are satisfied that they can still be 10 meter drop resistant, I think it likely most Pro Treks would be little different.
I certainly would be happy playing watch conkers with any of these!
Mitch