Definately two decisions, not one. So you actually like girl b? If not, you could break up with her, and go with girl a, but you'll only be annoyed you did when girl a goes to ibiza. If you do like her, stay with her, and don't touch girl a with a pole, especially your love pole. Of course, I'm using my own personal morals here, but realisticly, that should be your starting point to make your own decision from.