One of them hurts when it twats you on the head. That'll be an Arctic Tern.
One of them flipping hurts like a flipping numpty on steroids that's been forced into wearing really tight shorts all its life . That'll be a Common Tern.
For those of you with more sense than to live on a tern colony, I strongly suggest you watch it feeding....it's the clincher. Arctic Terns are a much more tentavive hunter than the more direct common.