Cooking surface– Panini grills come in three basic types, aluminum, cast iron, and non-stick. Aluminum and cast iron are more durable and will probably last longer but will require upkeep and seasoning. Non-stick won’t last as long, but your sandwiches won’t be sticking to the surface either.
Configuration-The thing that makes a panini a panini is the grooves-right? Without those it is just a toasted sandwich. But, the question is whether you want the grooves on the top and the bottom, or just the top. Grills come in both styles, the main difference being that a smooth service will give you a more even cook on the bottom.