Prospective is more certain than potential, but still not 100% certain. For example, after I sent my resume to a company and they looked at it, they called me because I was a potential employee. I wanted to work for them, and they were thinking about hiring me if the interview went well. Once they gave me an offer and I signed it, I became a prospective employee. I hadn't started working for them yet, but I was almost certainly going to work for them.