Leaving propane aside for the purposes of this subject,there are a couple of different styles of cooking a hog, and as pointed out here some of them are merely outdoor roasting but not true BBQ.
The cuban pig roasting box, which has the pig in a foil-lined wooden box with a steel fire plate lid on which you build a charcoal fire, uses a heat-induction principle to cook at a rate of about 15 pounds per hour. It is quick and convenient and actually turns out a pretty good-tasting hog, but no smoke ever gets to the pig, therfore disqualifying it as BBQ in my book.
The opposite extreme is pit roasting, which can take a couple of hours to dig the pit, 8 or so hours to burn a huge hardwood fire down to coals, then cooking the hog in the pit at a rate of less than 5 pounds per hour (usually somwhere in the neighbourhood of 24 hours). For purists only!
Most of us are somewhere in between, using a smoker or rotisserie and cooking at a rate of anywhere from 7-10 pounds per hour.
With regards to your initial comparison, remember that most hogs are considered done when the internal temp is in the 170*-175* range, while a butt is cooked to 200*