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Plastic does not create shorts when in contact with inadvertantly exposed wire. (You should take care to not allow wires to become inadvertantly exposed inside the box, anyway, but it provides "belt and suspenders" protection, just in case.) Plastic is often easier to work with, and you should independently ground everything to ground wires, anyway, not just the boxes attached to ground wires.
Metal boxes can be just as large if not larger than plastic. You are just judging the dimensions of the boxes in stock at your local store. I use deep metal boxes when I need MORE space.
Metal boxes (even gangable ones) come both with and without attached brackets for attachment to the studs.
You should not secure a box by nailing THROUGH the box from the outside edge. Secure the edge that is flush with the stud from inside the box, or (better yet) with a bracket that is attached/welded to the box. I use screws to secure boxes because you get finer control over where they end up. Use an extended screwdriver-bit (6 inches or longer) to be able to screw the screw outwards from inside the box.
Personally, I prefer the plastic boxes from a cost standpoint and the fact that there is more of the box pressed against the stud than a metal box with a 1" wide nailing flange.