GUCs expire, miles don't. If you have certificates already, and don't know if you're going to be able to use them on another trip before they expire, it may be worth burning now.

On the other hand, GUCs go to the gate, whereas miles don't (or don't reliably). If you can confirm right away and you're sure you'll use the GUCs on other trips, maybe go ahead and use the miles (depending on what else you might use the miles on, obv)