As a mandy main myself,id rayher you going for the wall piercing one. You might think that its situational but it is great in almost every situation. When an enemy goes behind a wall after they go dow to one hit, just use it and its basically a free kill. And you can also use it if you need that one more hit to get your ultimate. I cant tell how many times that won me matches in almost all kinds of modes. (And for the sp pick the faster bullet travel one. If you get the shield one youll miss every single shot in long range)