Stainless steel is the body of the piece. Gold plate is a colour on the surface. A good everyday piece uses steel as the body and a gold or silver colour as the look. A bad one uses a soft metal as the body and hopes the plate lasts the month.
Choose steel if you shower in your jewellery, sleep in it, or forget to take it off. Choose a heavier plate only if you already baby your jewellery and like rotating it.
See both in the catalogue: stainless steel jewellery and gold-plated jewellery.