Most caterers will tell you before you purchase a bite of food that you need to pay a minimum cost for equipment, depending on how elaborate you want things. Others will include equipment charges in the overall cost. Service - The wait staff, kitchen help and other servers. Whether it's a lavish buffet or casual picnic, appearance and timing are everything. Prices may differ dramatically, but probably not.
Caterers are a competitive business and all price out their services somewhat the same.
Your choice should consider style. Make sure your caterer is someone you feel comfortable with and trust. You need to share their taste and style. Food preparation is much easier in a familiar location. Try to find a caterer who doesn't include gratuities in the per person price.