Get the big one if cooking rolled or half slabs of ribs or shoehorning briskets between the 18.5" grate handles is gonna cramp your style.
IF NOT, this is my take on 'em: My 18.5" cooker is plenty sufficient and more convenient for my purposes, even though I regularly cook for 10-12 folks. If I'm gonna have a BIG bbq, pulled pork is gonna be what's served and four 7-9lb pork butts will feed many more than the Missus is gonna invite. Six slabs of ribs halved and cooked in two six-slot rib racks works great, although you can simply roll and skewer up to eight.
As for chicken, I'm not a stickler for "crisp" skin and will cook 10lbs of leg quarters on two grates or 20lbs if I use an extra grate. My favorite chicken cook now though is small 3.5lb butterflied birds, two to a grate with the legs pointed opposite directions and top of the breasts almost touching at the center of each grate. Briskets up to 14 or even 15lbs are no problem, and I've done several with two good sized 7-9lb pork butts on the other rack. Did that one night and had it all done in only TWELVE hours, water in the pan.
Bottom line, unless you're really serious about cooking for BIG groups quite often, go back to my first statement and let that be your guide. One other thing though, if you're a big foiler you might like the extra room of the big one.