I recommend booking it yourself and transferring the booking to Costco Travel, as they'll give you a larger credit than any other TA, in the form of a Costco cash card.
Also, since Costco Travel always has customer service reps available to talk during business hours, you can always get a person on the phone to talk about any changes you might want to make to your cruise, instead of having to wait for a callback within 24 hours or so, as you'd have to do if booking via one specific travel agent.