One of the age-old catering debates, particularly for weddings, is whether to serve a plated meal or a buffet. There are plenty of factors to consider, but a good rule of thumb is that plated meals are better for more formal events.
When it comes to your guests, the expectation they have is not that different from what they would want from a restaurant. If they dressed up to dine at an exclusive downtown bistro then they would of course expect the highest level of service. On the other hand, if a family goes out to eat in blue jeans then they would likely be more open to "serving themselves."
But it is not only service levels that makeup the final decision on buffets. For a full perspective you should check out Buffet vs. Plated Meals: What You Need to Know