Should i keep my sat subject scores?

As long as you are already going to retake one test, why not give history another go? Once you are already in the room on a Saturday morning, what do you really have to lose by trying again? It's not like you have to worry about scoring lower; schools only look at the higher score anyway. 680 is a respectable score, but it is only the 71st percentile. If I were you, I wouldn't turn down a chance to at least try to break the 700 mark.