Once you've worked with both for while, you will notice several differences. First, sterling wire is obviously solid all the way through. The plated wire has a copper core, so sometimes the finish can rub off, exposing the copper. You may sometimes also see the copper core after cutting the wire, like the inside of a rainbow candy cane if you snap it in half. Plated wire is generally half hard, where as sterling comes in anything from hard to deadsoft. There is a slight color difference between the plated wire and sterling... plated has a slightly more yellow tint (probably due to the copper under the surface), where has sterling wire has a more blue tint.
Of course the advantage to plated wire, it is far cheaper, but you will have to decide what you want to use, and what is best for your customers.