Molly Rae
I love charcoal but also different pencils can be fantastic. Pencils are easier in the sense of light to dark is easier + you can erase, but charcoal is incredibly forgiving so long as you don't go to dark. Try a little sample of both + see what you like best

DizzyLizzy
For transparency, I would prefer pencils. You can choose harder ones to have more subtle shades of gray. For metallic objects, same goes. It's the juxtaposition of really dark with really light reflections that gives the metallic effect. In general though, I prefer charcoal, cause it's more expressive.