bigger is typically always good.
but for conserving fuel(the larger unit will use alot more lump to run). The small WSM has plenty of capacity to feed a good sized group, it easily would do 20#'s of butts, and 4-5 racks of ribs using a rib rack. If someone needs more capacity than that the larger unit would be the way to go.
I am getting the larger WSM to do some side projects(possible competition run, etc.) that I dont want to use the smaller one for. Its only $300 for the big one, ive almost saved enough pennies to get it.
Since I think the WSM is the best smoker around I didnt even consider another type, just a larger model to handle a larger load.