All comes down to personal preference and what will be the best fit for you.
Did my first large-scale (for me, at least) cook for about 30 people. Smoked a 15lb brisket packer. Pulled it from the smoker and slipped it into the oven. Then put 12 racks of St. Louis spare ribs on the smoker. All ended up being done about the same time and I was able to feed this group with a ton of leftovers.
I don't plan on feeding big groups like that very often, but it's nice to know I have the horsepower should the need arise. I have the WSM 22.5. Size matters!