At the end of the day, Don't run out of food!
Let this be your mantra, repeat 20 times a day!
When talking with your catering professional be very clear that this is your goal. Now, on to some specifics.
It costs more to serve a buffet than a plated dinner.
It's a proven fact. You will have to order at least 20% more food AND your guests WILL take servings of all or most of the dishes; even both or multiple entrees.
You might have the "end of the line" with almost no entree and a plate of side dishes and salads.
The way to manage a buffet; have the entree served by your caterer and order 20% more food than you think you will need.
If these two things are done, you stand a very good chance that everyone will get to eat.
If you have over 50 people, consider strongly providing a 2 sided buffet, otherwise you may have people standing in line for a very long time.
Plated dinners are just that; number of guests=number of plates, period. The downside is of course if you have some late comers that you either didn't know about OR rsvp's AFTER the catering order had to be in, you don't have enough plates. Figure 10% higher; in most cases it will be eaten. One nice way to give a bit more variety in a plated dinner is to offer a duet; a smaller serving of two dishes. Sort of a surf and turf concept on a larger scale. The combinations can be quite creative and delicious. Ask your caterer to suggest some of their most popular choices.