I find with beavertails forends, the grip is one the forend, not the barrels, due to the width of the forend. I prefer the barrel grip as that puts my eye and sights on the target spot.

Gripping below the barrels on a big forend makes me want to raise the barrels above the target spot when I snap the gun to my shoulder. It takes time to then drop the barrels so the sights align - this slows target aquisition and isn't comfortable - for me.

Whether the gun has a pistol grip or not doesn't seem to matter.