I've always used Priceline.com to book my cruises and it's practically the same as Expedia.com I always check all these online booking sites for price comparisons before booking and they are usually almost always the same price with the exception that some sites might automatically include the taxes+fees while some will add it on when you are ready to book. That might be where the difference lies.
Also, sometimes they might automatically add the cruise insurance for you and if you do not want it you can opt to take it off.
And I also agree with previous comments, when you book through these sites, its best to call them and book it rather than online because sometimes certain things might be available over the phone and not on the website. But using the website to get an idea of the price and availabilities is a great option.