@Praks
GSM/WCDMA (Auto Mode)=Auto Mode means where there is 3G(WCDMA) then it'll catch 3G Netowrk, else 2G(GSM)
GSM Only = will catch only 2G (GSM ONLY) even though you have 3G tower (network)
WCDMA Only = only 3G (WCDMA ONLY) here you can get problems when there's no 3G then it'll show out of coverage area.
Hope you get the general idea, you can change the setting as per your liking but normally auto mode is ok, WCDMA ONLY and Auto Mode may consume more battery.
Switch to GSM only, if you get poor reception in WCDMA mode. Continuing in WCDMA will drain battery faster if network is poor,use this mode if you need it badly.
Entirely depends on how far you are from what tower.
Weaker signal means the phone has to use more power to amplify the weak signal.
If you want usage time then go with the nearest tower. Otherwise you will have to decide whether the increase in receive rate is worth it for shorter operating time.
Or go with another provider who can give you a better signal if your location is more or less static.