They are just different. Ask yourself how much in control of the negotiation you and your spouse want to be. Get the mediators to discuss their approach to mediation. It will help you become more knowledgeable about the process. These vary significantly from region to region, but a comparison of various mediator's fees in your area will be useful.
Most mediators require payment at each session.
Some will take a retainer up front. There may be a flat fee for preparation of the Memorandum of Understanding, the final document of the mediation. Clarify and compare. This is a great way to get to know the mediator and become familiar with the process. Not everyone offers it, but you should ask.