We had just the opposite experience. When I got a quote for our 12-8 Magic cruise, the
DCL website had the Fall Fantasy special listed, with the discount code and it was calculated immediately.
But when I emailed DU for a quote, they did not include FF in the price, so were much higher.
We booked directly through DCL. We had no problem talking to a representative whenever we wanted, or with adding the $50 postcard credit. Also, DU wasn't offering an additional rebate at that time, according to their website.
I'd say, go with what you're most comfortable with. I never use travel agents for any of my vacations, rather I prefer to search travelocity and other such sites myself.