That's obviously an option as well. To answer your original question, there's no difference in priority between a GPU and miles + copay. Miles + copay is rarely a good deal, but it might be worth considering if there's a big delta between the cheapest possible fare and the W or higher fare you'd need to use a GPU. The logic is, if you don't get upgraded, you get the copay back, whereas if you don't get updated with a GPU, you just get to fly on your (more expensive) W ticket.