I personally think customary units will give the pieces a stronger historical feel. Inches derive from the length of a certain king's thumb, from the tip to the first knuckle; a foot is, well, the length of one of his feet. A lot of them have "rule-of-thumb" origins like that, whereas metric is more modern.

Still drowning in coursework and character sheets btw. I haven't forgotten this, although by the time I start writing you might already have introduced Gen III.