In reference to the county or city? Lets use all ways people around you reference their locations. If they say South Florida, then use that. If they use Southern California, use that. Maybe they say Twin Cities, or Capital District. Whatever they use in that area in Google, is what you want to mention on your website.
All we need to do is have a success story, or an articles mentioning both or one. Another article, story or FAQ can mention the other locations. The goal is to have many locations mentioned on the site in a way that makes sense. So if a random person read random articles/success stories they would have an idea of WHERE you worked. Because you talked about all the different places around you.
Look at it this way. In 6 months if you have 20 success stories or articles on the site some can mention the same places (maybe most of your clients come from the same place) but others can mention OTHER places. So if we made a list of all the LOCATIONS mentioned on the website, we would have a good idea of WHERE you work. That is what Google does. It look at all the locations on your site and understand WHERE you work.