I agree with OBG, The worst thing you could do to a shotgun is put a pistol grip(only)on one. The shoulder stock with the pistol grip incorporated into it would not take away from the "multi-purpose" of the shotgun.
As far as Mossbergs go. The reason I prefer the remington over it is because the Mossberg has a alloy reciever housing and some alloy parts in it. Where as the 870 is all steel. However I think I remember hearing that Mossberg was switching to an all steel reciever or just recently did. In which case the 500 series Mossbergs would be just as durable as the 870's.... IMO