Hi Bobby, and Welcome to the party! 8)
This is the "age old" question, around here!! I started out with a #1, and almost immediately regretted not going larger. While it's great for a single pork butt, 3 racks of ribs, or a brisket (cut in half), it left me longer for more! I was hesitant, at the time, because I'd never heard of SI, and didn't want to sink too much money into an "unknown!" Had I known the quality of the smoker, I would have sprung for a #3!
So, the #2 is a fantastic unit, and I still use mine a lot. But, if the budget allows, you just can't go wrong with the #3! Believe me, when you want to stretch out some full racks of ribs, or that 18+lb packer brisket, you just can't replace the depth of the #3! You can always smoke less in a bigger smoker, but can never smoke more in a smaller one.
At least, that's my nickels-worth of 2¢, from a guy who has owned them all (except the 4).