Continuing on what EGOL said, we have had very good experiences with County + keyword for areas outside of Hillsborough County (outside of Tampa). You will not find Pasco + keyword or Hernando + keyword ever showing up on any mainstream keyword tool. They just don't have the search volume...however, we get conversions on those landing pages all the time.
Frankly, I don't know how you would even do county vs city research, because you (most likely) will not find search volume for counties even though conversions might be there. However, as William said, they are the same name. So, in your particular case, it really shouldn't matter.
Best,
Ruben