Everyone said above, but seriously do a little research on the type first, because a smoke that is big enough for one might require multiples in another, or might not even work in multiples in yet another. Seek the comps and type of comps first. Then think of how you cook, how you enjoy cooking, and frankly the type of BBQ you plan to present (they can taste and look different depending on the smoker).
For example, I have what is basically an older Lang 84. I can barely do shoulders and ribs of an MBN comp in it, and that's maxed out. I'd need another one to do a whole hog. But, for KCBS, this is plenty to do it all. For a backyard, I wouldn't bother towing this beast; I'd just use my significantly smaller Yoder knock-off. That stated, I obviously like offset smokers and the product they produce. With it comes fire tending; all night. Someone who wants to get some sleep would be better suited to a cabinet type of smoker...