That's not really true. Here's a brief overview of cellular technology:
1G: Purely analog. Useable for data only as a dial-up modem.
2G:
GSM - Widely deployed in most countries with cellular infrastructure.
CDMA - Widely deployed in the US, but not elsewhere.
Neither GSM nor CDMA is strictly better than the other. Wikipedia has a good overview of the advantages and disadvantages of each
here. In general, GSM is more flexible and open, while CDMA has better capacity and scalability.
GSM Data: GPRS
CDMA Data: 1xRTT
Both GPRS and 1xRTT offer data speeds (usually) higher than dial-up, depending on the variation of each that your provider and your phone uses.
3G (high-speed data):
GSM: HSDPA/HSUPA
CDMA: EVDO
The voice components are called UMTS and CDMA2000 respectively.
3G technology builds on 2G to offer broadband-level data rates. Generally speaking, the throughput on these networks peak around the speed of a low- to mid-range DSL or cable line.
4G (HIGH high-speed data):
GSM: LTE
CDMA: UMB
4G technology is not yet widely deployed anywhere, but (at least in theory) offers bandwidth well in excess of normal broadband speeds. How well it will deliver on that in practice remains to be seen.
Now, returning to your point: NextG is actually not a change of generation. They're just switching from the CDMA family to the GSM family, without changing generations. UMTS and CDMA2000 simply have different things that each is better at. UMTS is generally better in hilly terrain, while CDMA2000 generally has fewer dropped calls under "normal" conditions, etc.
Any claims that "NextG" (and, in fact, the name itself) is better is purely marketing spin.