The coldest it gets here at my location in NorCal is at night, and it will occasionally drop into the teens. Upper 20s and low 30s are normal in the winter. I've smoked in the 30s, wind blowing, no insulation with my WSM. The advantage of a vertical smoker, with the fire below the meat, not offset, is that it maximizes smoke chamber heat retention as the heat rises. I've never felt the need to insulate my WSM given my conditions.
The real question is how much money do you want to spend on a fire-below vertical system? $300-400 + $30 for a roomy WSM, welders blanket and some clips, or $300-1200+ for a slightly less roomy but insulated Kamado system you can also use as a grill. I don't think you can go wrong with either.
I've never used a Kamado system, but three people I've known who owned one could never get the hang of low n slow temp control to smoke with it. They weren't patient or analytical people though, so I chalk it up to personality, not the system.