Maybe Princess has spoiled me, but I prefer booking with the cruise line. It's the same price. All my information and preferences are saved in their website. I am assigned a cruise specialist through them who contacts me and helps me before the cruise.
Putting too much faith (and money) into a person or agency I don't know, scares the dickens out of me.
Granted, I do my own research online, I buy books, and check this board faithfully.
On board credit, future cruise credits, and credit card rewards get me many freebies.
Perhaps I'm lucky, but doing my homework ahead of time guarantees me I will always have a great trip.
Everyone has their preferences, this one is mine.