2G GSM uses a form of "time division" (TDMA) to keep concurrent
calls from interfering with each other.

3G GSM (or "UMTS") uses a form of "code division" (CDMA) to
achieve the same end.  UMTS is often referred to as W-CDMA to
distinguish it from the older CDMA systems.

CDMA systems are not distance-limited by timing constraints.
Normal 2G GSM cells have an absolute maximum range of 35 km
(regardless of signal strength).  Because of the density of
cells in populated areas, this limit does not usually become
significant.  But it can seriously affect ordinary GSM coverage
in sparsely-populated rural areas.

The best system will often come down to a matter of which system
has coverage in the region where you want to operate.

3G GSM (by whatever name: UMTS, W-CDMA) is the way of the
future.  This is where the explosion in handset availability
and features will occur.

John

I like the fact that I'm free to use any unlocked phone with the SIM
for my account without any intervention by my carrier [assuming that
the phone operates on the bands my carrier uses].

In the U.S., if you're traveling to rural areas then go CDMA as you'll
get much better rural coverage than GSM because the maximum GSM distance
is limited to 35km.

Actually there's more to it than just GSM versus CDMA, you also have to
look at 800 MHz versus 1900 MHz. The best choice, in terms of the best
coverage, would be a carrier that is both 800 MHz and CDMA. This is why
Verizon and Alltel are consistently ranked as the best carriers in every
metropolitan area in the U.S..