I can answer this one. My wife works for Cingular Wireless. And I'm a level two data and voice technician for Sprint (for a little while longer).
Cingular Wireless has better coverage (nationwide), but Nextel has better service. Now, Cingular Wireless just bought AT&T. If that gets approved in November, they will have the nation's largest network.
However, Cingular is also in severe cost cutting mode. The 41 Billion that they have invested has put them in financial straits. So, they are not building any new towers.
Where as Sprint is building 1500 towers next year alone!
Currently Sprint has more towers than any other carrier, followed by Verizon, AT&T, and Cingular.
Nextel is one of the smaller cell phone carriers. There customer service has recently been rising but they are ranked pretty badly. The only company's ranked worse are Cingular, AT&T, and Sprint.
Also, Two-way walkie-talkie is becoming outdated, as Voice over IP becomes the fad. Stay away from TDMA all together if possible. Go with a CDMA carrier and you will have better options -- especially when you?re in the hospital.
Hospitals are "restricted zones" for towers. So, signal will always be poor there. The key is to get a phone that can operate on poor signal through data compression. Sanyo's are coveted for this, as are Sony phones.
Hope that helps.