Pretty good advice from everyone.
Not only do you need to compare prices and bonuses or perks, but most importantly, you need to know the boat that is being used and the cabin location being offered. There are good reasons why some cabins in the same price category don't sell quickly. This is why a $600 cabin from TA "A" on boat B is not the same as the $600 cabin from AA or UA with the same boat.
It takes a lot of work (i.e. time) to go through all the variables for booking cruises, but I agree that working with a live agent is much easier than a keyboard.