Everybody I talked to that bought a 60 said they wished they had bought the 84. So, I bought the 84 and it is perfecto!!!
Go with the biggest you can afford. I wish I had bought the twin 84.
It is only my opinion, BUT, if you buy good meat, use a good rub and have a good idea how to use a smoker, you DO NOT flavor enhancers!!!
I may not know much about BBQ, it's just the way I do it.