Basically, a whole pork shoulder is broken up into 2 sections.. the boston butt and the picnic. Usually when you eat pulled pork in a restaurant or you hear folks talking online about pulled pork they are referring to the boston butt which is the upper part of the pork shoulder.
It is extremely easy to smoke and very forgiving to new smokers becuause of the nice marbling of fat throughout the meat that helps keep the meat moist during long smokes.
I personally feel that the butt has a better overall flavor than the picnic but that again is a matter of preference and will vary depending on who you talk to.
I rarely see a whole pork shoulder around here so I have not done any price checking of butts vs the whole shoulder but it does seem odd that it would be that wide of a margin between the two.
Maybe others in here have noticed that as well and can shed some light on the reason for the difference in price.
For the most part.. if you have the opportunity to buy the whole shoulder then it would be an excellent smoking experience as well as a good way to feed the neighbourhood.