GSM/WCDMA Auto, or GSM only setting on Android phone ?

Under Android 4.x variants, if you check under Mobile Network settings, you may notice "Network Mode" with the following possible options (not all may be visible in all devices):
GSM/WCDMA (auto connect) - automatically switches between 2G/3G modes. When signal strength is low, 3G is favored more
GSM only - 2G only mode. When 2G signal strength is low, you get no data.
WCDMA only - 3G only mode. When 3G signal strength is low, you get no data.

Less common/other mode variants are listed below:
WCDMA preferred - Same as "GSM/WCDMA (auto connect)". Both 2G and 3G data capability, 3G favored more when signal strength is low
GSM auto (PRL) - 2G is favored more when signal strength is low. Both 2G and 3G data.
CDMA auto (PRL) - both 2G and 3G, 2G (CDMA) is favored more when signal strenth is low.
CDMA only - 2G data only, no data when 2G signal strenth is low.
EvDo only - 3G data only, no data when 3G signal strength is low.
GSM/CDMA auto (PRL) - auto-switching between GSM and CDMA capabilities.


In a nutshell: GSM = 2G, WCDMA = 3G.
GSM uses less power and has limited data speed (~144kbps)
WCDMA uses more power and has good data speed (~2+Mbps)

For auto-switching between 2G/3G modes, use the "GSM/WCDMA auto" mode (system balances speed vs battery use based on current signal strength). For 2G-only battery saving mode, use "GSM only". For higher data speeds (regardless of battery use), choose "CDMA only".

WCDMA (Wide Band Code Division Multiple Access) or UMTS (Universal Mobile Telephone System) is a third-generation (3G) mobile wireless technology - this standard has been developed to accept higher rates up to 2 Mbps (local area access) or 384 Kbps (wide area access). To obtain higher speeds, HSDPA (high speed downlink packet access) was introduced in version 5 of the WCDMA (3GPP). HSDPA allows data rates up to 10 Mbps (20 Mbps with MIMO).


Notes:
Above settings may be accessible by hitting *#*#4636#*#* on your device and selecting "Phone information"
You may have to add/edit your phone's APN settings to match your network.
If the network mode you're trying to access is not currently available at your location, or its signal is weak, the Network mode setting may not be available (depending on your phone/model).