would?could?

Both of them are polite and appropriate if being used in your example.

However, -could- has a little bit more on the commanding side, while -would- is more suitable when asking for permission from someone.

Take your example, if it is the customer asking the waiter to do something. He can use both -could- and -would- in his request.
He is eligible to use -could- because basically he is kind of requesting the waiter to do something for him, not asking for his permission, therefore using -could- is still appropriate for the customer.

On the other hand, if it is the waiter who is asking his customer, it would be better if the waiter use -would-.

e.g.
Would you like some coffee, sir?
or
Would you mind having someone to join your table? Because the restaurant is quite full at the moment.

Waiter should not use -could- in such conversations because the customer has the right to reject his suggestions (not request).

If the waiter says -Could you give me that plate?- when he is collecting the dishes after the meal.
That would be kind of rude.
It would be better if he asks -Would you give me that plate?-

could:can的過去式 (用於婉轉語氣)能,可以

would:will的過去式,表示過去未來式 (表示婉轉的請求)請