For 30-50 people it wont be a problem on the WSM, you can do 6 or more butts on the WSM. I would do the butts on one day and the chicken on the day of the event. Just chop or pull the pork when its done and put it in food saver bags. To reheat it, just drop the bags in boiling water for 25 minutes. Keeps the pork moist and just like it came off the cooker.
I usually figure 15 sammies per butt. I would do 2-3 butts for that group.