Do petunias grow better in sand or soil?

Sand is rarely a smart cultivating medium for flowering plants, unless you're growing cactus.

Sand, does, however, provide a good drainage option when covered with topsoil deep enough to support the plants you want to grow.

From About.com:

"Petunias will tolerate any range of soil pH. They don't like to be dry for long periiods, but they also don't like wet feet."