Playing on different lines and in different situations. If one was replacing the other your argument might make sense. Also, +/- is such an arbirtary stat that all 6 guys on the ice contrubute to.
That being said, Robertson is the more talented player for sure. More effective? Not sure. If Reaves is not scoring he can hit, fight, create some energy. What else can Robertson do? I hope he has a great game and finds a way to contrubute every game. So far, he hasn't proven to be any kind of a difference maker.