I would set it offline. I say this because SINGLE_USER mode is generally used to allow an administrator to perform some kind on maintenance on a database without users attempting to access and possibly interfering with the maintenance.
It doesn't really matter either way for a restore as your users will experience the same thing which is they wont be able to connect. One possible annoyance with the SINGLE_USER approach would be if a user stole the single session before you and blocked you from performing your restore.