One piece and sets are all about convenience. Quick to get on and off so you get to your destination dry. At the lower price point PVC is used which is highly waterproof but not breathable. Spend a little more and Polyurethane will breathe so you stay cool as well as dry. As a general rule of thumb, the more you spend the better the product performs with regard to waterproofing and breathability.
A very high quality set of waterproofs£
A set of waterproofs for long distance riding£
Entry level set of waterproofs£