There are two trade-offs in choosing a medium to make a design in. The first is the time it will take you to make the design. The more faithful the design is to the real application, the longer it will take you, even if you mock something up quickly in code. When you are early in the design process, making your designs quickly so that you can get feedback on them before you invest too much time in them is ideal. When you are later in the design process, making your designs higher fidelity can be helpful to refine details and get specific feedback from users.