"Cattle boats" here on Guam are boats that primarily have vacation tour groups on them. They normally go to the same sites day after day, and most of the divers on board are of the "dive once or twice" a year variety. The divers, for the most part, are clueless (with some exceptions) and each group has their own divemaster to baby sit them. Our shop does rent spaces to tour groups on every morning trip...but they also have a "local" boat that goes out in the morning. That boat usually is packed full, but the difference is in the divers. They know the basic rules, don't need baby sitters, and have respect for other divers on board.
It all depends on where you are diving. I went on a "cattle boat" in Yap, and everyone there was very professional and knew what they were doing. There were at least 30 on board, and we all went to the same sites, but everyone gave each other space.
I don't know if this helps, but don't rule out big boats. Just ask if they have tour groups on board. Like Natasha said, the big boats have the most advantages. And, from what I understand...live-a-boards moor to one spot for hours. That mean everyone doesn't jump in the water at the same time. If you want a divemaster, they will provide one. But, anyone can jump in at anytime if you feel comfortable doing so.