It's not so much the CARRIER as it is the TECHNOLOGY. There are really only 2 types - GSM (SIM Card) & CDMA/PCS/EVO
From what I've seen, CDMA/PCS/EVO phones work in more places than GSM (SIM card) phones.
HOWEVER, that can vary. CDMA phones work for crap in Lingle, while GSM phones run full bars. OTOH, the converse is true in Torrington.
It's all about tower placement. And while that DOES vary by carrier, from what I've seen with carrying both forms of tech, the CDMA (Verizon/Alltell/???) has fewer overall dead spots than GSM (AT&T/???).
Find out what tech the carrier uses and you will have a better clue as to coverage.