It's really up to how comfortable you are with using someone that isn't really there! Oh, I know most of the on line agencies will assign you a live agent, but it isn't like booking with your hometown TA. The other side of the coin is where will you get the best deal, how many times have you cruised and what do you expect. I do like the idea of using an agent in case of a problem. You have someone (hopefully) on your side. We do still use the company I used to work for, but next time think we will try the internet. It's like booking your tours on your own versus using the cruiseline. Most of the time you will be better off on your own, yet, some are not at ease doing it that way.
good luck and let us know what you decide and how is works.