Having been doing this for 11 years and having clients all over the world, I can honestly say that I'm not a big fan of online sites. Most of the time, if you do have the occasion to talk or email someone, they are not certified travel agents. They are simply people who work in a call center and their job is to sell. They usually have not even been on a cruise or to any of the ports, so their expertise is extremely limited. It's all about quantity to them and not quality. I've gotten alot of complaints about two of the three agencies you've mentioned for just that reason.
Anyone can sell, but it takes a good agent to provide good service.
It's best to work with a reputable agent who will take the time to personally talk to you and find a cruise that best fits your needs, budget, and lifestyle. After all, they want you happy so that you'll use their services again.
Best advice is get referrals from those you trust, then talk with the agent so you can get to know them and they get to know you. Ask alot of questions! Many travel agents are not cruise specialists - they only sell cruises and haven't experienced them firsthand. Find out how many cruises they've been on, how many different ships, and what ports they've been to. You not only need information about the cruise line and ships you're considering, but you'll also want to get information about where you're going, what to do, what not to do, etc.
The agent should never be in a hurry to answer your questions and be more than willing to spend as much time as you need to help you find the best cruise for you.