First, I don't mean to cut the #2 down, because I think it is a great average size smoker and you can smoke a LOT of meat in a #2!
Here are just a few more observations about the differences:
I think the 35 verses 45 pound capacity is kind of a worthless rating. How is this calculated? I think a more effective measure would be usable space verses pounds. The length of the #3 gives you the capability to smoke a variety of meats whole verses having to cut them down in a #2. Admittedly, this isn't a huge deal. But, I prefer to cook most cuts whole instead of cutting them.
When cooking like pieces of meat I prefer to cook them on the same level. For example, you will get a lot more space between the meat if you want to smoke two pork butts on the same shelf. You "may" be able to do it on a #2, but they are going to be pretty close to each other and could affect heat flow.
Again, I think it is all about usable space. Here are the available square inches of the 2, 3, and 4.
#2 - 4,500
#3 - 6,600
#4 - 9,720
Technically, I could buy two #3's for less money than a #4 right? I would even have a lot more square inches. But the nice thing about the #4, is that you get additional height so you can do longer cuts of meat or even hang sausages and the like. Likewise, with the #3 verses the #2, you gain usable square inches lengthwise where you can fit larger cuts of meat instead of cutting them down.
The other thing is leftover smoked foods often increase in flavor after they have rested in the fridge/freezer. I always smoke more than I need and vacuum seal and freeze the leftovers. There is nothing like being able to pull a vacuum packed back of pulled pork out of the freezer on a freezing cold day where I don't have the time or feel like smoking in the cold.
OK, I will concede that if you plan on traveling with your smoker, the girth and heft of a #3 could be a factor. I have loaded my #3 by myself, but prefer not to. It is super easy with two people, but tricky with one.