Pro: In the hands of the right baker, fondant helps to create some of the most epic and aaah-inducing wedding cakes.Most "sculpted" cakes are frosted mainly with fondant, and even simple fondant-based accents can look incredible.
Con: To some people, fondant is simply not as tasty or texturally-pleasing as buttercream. Poorly executed fondant can also sometimes have an overly fake or plastic appearance.
Pro: Most people agree that buttercream is the more traditional frosting; people generally prefer buttercream in taste and consistency. Buttercream also produces gorgeous cakes, particular in styles that are softer or more traditional than fondant.
Con: If you are looking for a highly-sculptured or intricately designed cake, buttercream's texture isn't going to be able to accomplish all that fondant can.