A solid plastic cup is at least reusable. Styrofoam cups are always single use. Breaking down faster is what makes them worse.

All “break down” means here is to turn into extremely small micro / nano particles. With hard plastic the damage you’re doing to the environment is more constrained to one piece which you can scoop up if you want. It’s way worse if it’s turned into dust and distributed in rain water, ocean water, the veggies you eat, etc.