It all boils down to what you need vs what you want, somewhere in the middle is your compromise of what you choose.
1) First let us consider size.... Basically a two-fold question because it involves overall size as well as the inside capacity.
Are you looking for a large tow behind smoker or a patio model? With smokers as well as many other choices, size is important, but bigger isn't always better. Larger smokers have more fuel consumption and may require more time tending them. One should never consider a larger smoker just for show. Practicality should always be the first and foremost thought in this equation.
For backyard use, one typically looks for a smaller patio model. Some competitors also use smaller models in smaller competitions for the ease of portability and mobility. Other competitors have much larger cookers because they also sell to the public as a vendor which is allowed at some national competitions as a means of recovering some expenses as well as selecting the "People Choice" award. At National Competitions like the "Best of the West Nugget Ribs Cook-off" in Reno, Nevada it is common for some of the competitors to cook over 1500 pounds of ribs per day. If you are looking to compete start with something small and you can always move to larger units as you progress.
Wood Burners, Pellet Smokers, Charcoal Smokers, and Propane units all come in patio/portable sized models. But will the patio sized model hold the capacity you are looking to cook. This brings up the issue of Horizontal vs Vertical models. Some horizontal models can be fitted with a second smaller shelf to allow a little more cooking space, where as the vertical model come in varying heights to allow more shelves for more cooking area.
2) Second item to consider in choosing a smoker is the type of fuel you wish to use; Wood, Charcoal, Pellets, Propane Assist, All Electric, or All Gas. The obviously simple set and forget smokers would be Electric, Gas, or Pellet smokers. With electric and propane smokers you only have to be concerned with an ample amount of wood chips / chunks for smoke flavoring. With the pellet smokers, some owners say there isn't enough smoke flavor, but I have had food from a pellet smoker with great flavor. Also on long cooks when using the pellet, one has to refill the pellet hopper to keep the unit going. Also if you plan on competing at higher levels like the KCBS, Propane and electric smokers are not permitted.
If you are considering a wood burner consider your available fuel supply. Are you going to have to buy a lot of wood and are you willing to stay up all night tending a fire? This is not meant to discourage you from a wood burner, but to simply find your level of commitment to your smoker. Many people who use them enjoy the solitude of having a beer and a cigar as they relax in the quiet hours tending the smoker while the others sleep.
If you are considering a charcoal smoker I would suggest an insulated charcoal smoker. An uninsulated smoker burns fuel at a rapid pace because the heat is quickly lost and not held inside the smoker, whereas the insulated smoker burns at a much slower rate only to maintain the temperature within the smoker. I have seen some insulated smokers burn for 16 hours with a single five pound load of charcoal.
Once you have chosen the type of fuel you wish to use, we move on to the next criteria of choice.
If you plan on catering, check with your local health department. Some (not all) municipalities require a NFS rated smoker which limits the playing field to a handful of available units. It's best to find this out before you buy rather than after.
With catering in mind, visit your local Health Department can offer you a wealth of information which can help you before you have to select your purchase. Different States, Counties, and Municipalities may all have different requirements and regulations.
