Conductor and Dielectric. Further more there are a bunch of presets for all kinds of materials.
In my opninion the Presets should be in a single list, since it's not always known to which type of Fresnel the belong. Now whe have to look in both types to find a specific preset.
E.g. Mercury shows up only when Conductor is selected.
I think Preset must reside above Fresnel (type/model).