Personal preference, but I feel like unsubscribing or hitting spam is you sending your sentiment back to the sender in two different ways.
If you unsubscribe you're saying that you're no longer interested in the list, you see the merit but don't want to receive it any more for whatever reason.
If you hit spam you're saying this email should never have come to me, or I don't want to expend the effort to stop it from coming to me.
If the sender makes unsubscribing as easy as hitting spam (by making sure the Gmail unsub button works for example) then they make it more likely for their recipients to send the appropriate feedback - ie not hit the spam button.
If you unsubscribe you're saying that you're no longer interested in the list, you see the merit but don't want to receive it any more for whatever reason.
If you hit spam you're saying this email should never have come to me, or I don't want to expend the effort to stop it from coming to me.
If the sender makes unsubscribing as easy as hitting spam (by making sure the Gmail unsub button works for example) then they make it more likely for their recipients to send the appropriate feedback - ie not hit the spam button.