Having both would be a good thing! If limited, mid-grey is the hands-down winner. Charcoal can look funny with navy, having that just-missed look. I got my mid-greys from BB's winter sale and have been very pleased. BillThe mid grey flannel has a little texture and subtle variation. The charcoals are more uniform.
I am thinking of wearing with navy odd jacket, and tweed jackets and brown shoes. Both are Polo flat fronts.
Or perhaps I need both!?