We've always just booked directly with
DCL--and at first Costco sounds very tempting to do for our next OBC when we cruise next month, especially since we're executive members. But I don't know that I care for having to wait until travel to get the cash card. And then once we have it, only able to use it at Costco (although I suppose you could put money towards future cruise?).
The TA route, however, seems more personable, and when we've booked a WDW vacation, we did enjoy all the extra little perks the TA threw in. I know my parents TA gives them a ton of extra OBC since they've been loyal repeat customers. And I tend to feel more secure knowing that I can call so-and-so and ask if such-n-such has been added to our account/request made and build a trusting relationship, whereas with Costco, you have to trust that messages get passed on and completed, with no real person to take accountability (I know that a lot of people on the board use Costco and are
very happy, this is just my opinion and neuroses

).
So, I'm not clear on who I'd use at this time either! Although I do have a question: can I name a TA when making an OBB if I didn't use a TA for the cruise I'm currently on?