A metal shed, as said above really changes with temperature, so hot in summer and really cold in winter. Plus I think it would be pretty difficult to modify, i.e. shelves for the bunnies to climb on, or put supplies on.
Plus with a wooden shed you can easily add insulation to help better protect the buns in winter. Some have even added heaters, which wouldn't be possible with a metal shed.
Are you thinking of changing your set up?