I did my entire shop with metal boxes and conduit. Reason one: all my wiring is on the outside surface of the wall. The number two reason has proved to be a very close second - flexibility. You can run more than one circuit in the conduit sharing a ground. Also, I've had reason to add another circuit from time to time and was able to pull additional wires where needed.
Given you'll be running romex inside the wall, the advice you've received so far is sound. Metal in the ceilings and exterior walls, plastic inside.
“We all die. The goal isn't to live forever; the goal is to create something that will.” - Chuck Palahniuk