Most, if not all Cell service providers have a coverage map on there website. Be careful however, the maps may not be accurate. If possible explain what kind of coverage you want and where, then take a phone for a test drive.
Service here at my house is spotty sometimes. The sales staff at Verizon made every attempt to make it work. When it didn't work they were very understanding and didn't give me any trouble when I bailed out of the contract after a month of no service at home.
I had to go back to AT&T. As much as I don't like them, they are the ones who have been able to provide me service here at home. I'm typing this reply out in the yard, near the creek, while my laptop is tethered, (wirelessly) to the cell phone, (10 yards away?) in the house.
Cheers!
Ferrous