Check the cruiseline websites (and other travel websites) and local Travel Agencies to compare prices and perks.
I went with a local TA for our last cruise (Carnival Conquest). She set us up as a group booking and we were able to get a lower price than we could have got on our own (I looked). Each cabin also received $75 shipboard credit, $15 photo coupon, chocolate/fruit tray, and champagne or sparkling apple cider (kids cabin). If we were able to get 8 cabins, we would have recieved one berth free (we would have split the $ among all cabins). We ended up with only 6 cabins, but were able to keep all the perks and the low pricing.