the problem with insulated 2 by 6 walls, besides the added expense. is that by the time you get cooking,some type of lights and a heating stove, body heat. you end up cooking yourself out of a smaller structure.
2 by 4's are plenty good enough. a simple birds mouth rafter design is the cheapest/effective/less material for smaller structures. and allows for storage above the ceiling.
a pic of one i built a few years ago, from salvaged lumber,windows. cost less that 500$.
another one built from new material. cost about 1100$. with a truss design(wouldn't do that again). birds mouth is the way to go.cheaper.
i like small cabins as there easier to heat,maintain.