One of the downsides to switching carriers is always that your service could even be worse with a new provider than than the one you have now. Since whether or not you'll get coverage in a specific place also has a lot to do with your phone and how many people in the area are also trying to make calls at the same time, there isn't a foolproof way to know for certain whether the grass will actually be greener on the other side. But if you are thinking of switching, there is a site called DeadCellZones.com, which is worth checking out. They're trying to keep track of all the gaps in cellular coverage, so you can see if other people have reported problems in your area.
Read - DeadCellZones.com