There are two common styles of photography.  Classic wedding portraiture uses posed shots of individuals or groupings in which lighting is controlled and modified to soften coloring or create a dramatic effect.  Photojournalistic photography is an unrehearsed recording of the day.  The photojournalist moves in for a shot and moves out again unobtrusively.  People are unaware of the camera and the natural look shows things exactly as they happen.

Start interviewing photographers early.  Line up prospects and make appointments to see actual samples of each photographer's work.  Look for sharp, clean images that are technically superior and show genuine emotions and relationships between subjects.