Personally, I would do neither. A Phaser instance on the server is overkill. From my understanding, you only need simple 2D collision detection (circle-circle, circle-rectangle, rectangle-rectangle, etc.), and simple physics (2D vectors, trigonometry, etc.). Phaser will add so much weight to your server (i.e. use up memory) meanwhile you'll only use a fraction of it. A standalone physics library is overkill. Again, you only need simple physics and collision detection. Why not try writing your own implementation on the server?