I think you may be wrong.
Seeing as the settings panel on iPhone literally says "Using 3G loads faster, but may decrease battery life" is enough to make an argument.
From Apple's Support website: "Turn off 3G (GSM model only): Using 3G cellular networks loads data faster, but may also decrease battery life,
especially in areas with limited 3G coverage. To disable 3G, from the Home screen choose Settings > General > Network and set Enable 3G to Off. You will still be able to make and receive calls and access cellular data networks via EDGE or GPRS where available."
Especially shows that while you're correct that it will drain more under poor coverage areas, it doesn't exclude areas with greater coverage from still having shorter battery life.
http://www.apple.com/batteries/iphone.html
I don't think I deserved a minus on my post just for wanting to observe the way people use their phones.