I don't give this site much credibility. Most of the people giving reviews don't know anything about how cell phones/wireless networks work. Their tower locations are also very inaccurate, not even showing half the actual towers. I'm 99% sure it just pulls them from the FCC database. Bottom line: don't go by what people say, judge a carrier by your own experience and get what works.
It seems like sprint has about 60% of the towes at&t does in my area, but still works fairly well (still some holes I'll admit, but I think NV will fix that greatly). I've had access to at&t's internal mapping software, so I've seen their towers.