Either will work, and no need for sealant. If you do use sealant I would use paste and very minimal... it is the intake manifold and there is a small risk of bits getting pulled into the engine.im getting ready to plug the relief valve myself. Does it matter which type of 1" NPT plug. Theres two types at my local hardware store, black iron and galvanized. Would either work with a little thread sealant?
I used black iron and a touch of paste... mostly to help protect against dissimilar metal corrosion (I'm not sure what the intake is made from).