Boy, that's something that is going to be very subjective, based on what you're looking for. I'm not sure how much service you get from Costco. I've seen people say that if you call Costco Travel for
ABD, they actually transfer you to ABD. But you still get the discount. Not sure how it works if you're physically at Costco Travel (do they have brick-and-mortar Travel agencies?). A lot of TA's offer rebates. I'd check out several options, and see what they offer you. I think most will give you a no-obligation quote (I know DU does).
Then make the decision that makes the most sense for you.
Sayhello