Depends on the guy/gal ratio (guys eat about twice what gals do). If it's about 50/50, then two 6lb. butts will feed about 24 peeps (per work party wife my recently had that I cooked for). That was just enough.
If it was me, I'd do the 2 butts and only one smallish brisket.
P.S. If I am unsure about how many peeps will show, I usually bring a "bonus butt" and don't warm it up unless I'm about to run out of everything else(although I rarely do, since I'm pretty good at judging the meat/peeps ratio).