The difference is in the challenge to the design of the floatation vest. A Type III vest is more comfortable, and therefore most likely to be worn. However, the amount of floatation provided, and the positioning of the floatation material means the probability of keeping your head out of the water is compromised over the Type I and Type II design. Type III pfd's are considered floatation aids and will require the wearer to put in effort to keep his or her face up.
Most people who drown in boating accidents are not wearing personal floatation devices. Most people who drown while wearing pfd's did not have them on correctly (did not close the zipper, snap the snap, or have a between the legs safety strap in place). Wearing a loaded fishing vest over a comfortable pfd would probably fall into that category.
Look for a fishing vest/pfd combination which was designed to allow you to pack stuff into the pockets, and still allow you to right yourself and keep your head above water.
I did see a vest so designed in a recent Canoe and Kayak article on paddling pfd's.