Great question and I can only partially answer since I have not cooked for 100 yet. I would do whole chickens, it will probably save a lot of money doing wholes. The pulling wont be very fun, but chicken pulls pretty easy.
Make sure you cook to 165. Chicken will be harder to pull if it cooks too long and dries out.
Also, save your drippings from some of the birds, then mix it in with the pulled chicken for added juiciness and depth of flavor.
Take some pics. I am interested to hear your results.
2 - WSM 18.5, 1 - WSM 22.5, and 1 - ECB Smoke n Pit.