Both if you are serious, ear muffs if you can use only just one. Sound is difficult to block completely, even if the earhole is completely blocked, as vibration to the inner ear still occurs through the skulll, bones around the ear and teeth via bone conduction. This is also why two sets of protection you dont double the protection rating. 30Db muffs and 20db plugs dont give you 50DB protection, you add 5DB to the highest of the two, so its only 35. Over the ear protection with good 'clamping force' will reduce some of that conduction to the bones around the ear at least. The fact is experts cant decide how effective ear plugs ratings are for guns, they are mostly designed for non impulse engine noise.
One of the biggest ear killers is gun ranges as you are absorbing all the fire from people around you. You may only fire so many rounds plinking in a lifetime, at a gun range you can absorb several lifetimes of noise in a few hours.