Whole grain and stone-ground mustard are both coarse and will add a bit of a grainy texture, whereas the standard bright yellow American and Dijon mustards will not.
Dijon and whole grain are commonly found in mustard mashed potatoes recipes, and some even call for both. It's hard to imagine going wrong with any of them.
Enjoy mustard mashed potatoes with turkey, cranberry sauce, sausages, asparagus, or red meat. Consider adding sautéed onions and steamed cauliflower, as well.