What's the Difference between all of these amplifiers? How do I know which one to get?

Cingular/ US Wireless:
Can be on 2 different frequencies.  For the regular cellular service which is TDMA/AMPS, this is mostly the 824-896 MHz regular cellular band. This signal is throughout the country. In places where Cingular is not the main provider, they have agreements with the other carriers to bridge the area,  A single band amplifier will work. However, the Cingular GSM service can be on both the regular cellular band and the PCS band. In fact, in most urban areas, the PCS signal is used. In areas where there is dual service buy the Dual Band amplifier DA4000 . 

Verizon / Alltel
These services use a system called CDMA, which operates similar to the way the internet operates. These high speed systems require greater quality in the amplifiers. Alltel is only in the Cellular Band range (824-896 MHz). Verizon is in both the Cellular band (824-896 MHz) and the PCS band (1850-1990 MHz). Verizon is mostly PCS in Florida and Cellular everywhere else in the United States. They are in Both the Cellular and the PCS band in most major cities as well. Generally speaking, if you are in a rural area, you are in the cellular band. They also have agreements with other cellular carriers to bridge any areas that they do not have a tower. The DA4000 is used for the Dual Band.

Cingular GSM
The system operates similar to CDMA.  The  DA4000 works for both bands.

Sprint
The Sprint service uses another form of CDMA similar in nature to the Verizon system.  Sprint has 2 bands that they use, the Cellular band (824-896 MHz) and the PCS band (1850-1990 MHz). On the cellular band, in the rural areas of the country, the Sprint system operates on Analog (AMPS) system. In almost all cities in the country, Sprint is on PCS. You can also use the DA4000.

T-Mobile T-Mobile GSM
This service is GSM, which is the standard used mostly in Europe and Asia.  They use the PCS band (1850-1990 MHz). Overseas, these systems uses other frequencies not found here in North America.  You can also use the DA4000.

Sierra Wireless Air Card:  We have used the DA4000 in PCS mode. 

Can I use the Boosters as a Wireless Building Repeater system?
No. The boosters are made to connect directly to a cell phone and therefore are not sensitive enough to pick up a wireless transmission from your cell phone.