The Atlantic City Jitney is inexpensive and runs 24 hours a day. If you'll be venturing further afield to nearby attractions like Cape May and Storybook Land, though, you'll likely need a set of wheels. Renting a car isn't necessary if you plan on spending the majority of your time between the beach, the boardwalk and the casinos. Parking is plentiful and surprisingly affordable at most casinos.
Cars are relatively convenient in Atlantic City, and you can find rental agencies in both airports.