Mortar & pestles are bigger, great for coarsely grinding spices and making pesto, chimichurri, guacamole or your-favourite-condiment. Spice grinders are smaller and create a fine powder, so if a recipe calls for cumin or fennel seeds, these help.
Cast iron is great for hard or fibrous ingredients, porcelain is good for nuts and herbs. Both are easier to remove flavours from previous recipes. Choose olive wood if you are planning to use it repeatedly for the same purpose.