This is just an observation from my point of view. I'm sure it varies a lot from person to person.
I am seldom in a 4G spot. My previous phones sucked battery when using WiFi, so I used 3G all the time on the Thunderbolt. I experimented for a few days with WiFi, and found the Thunderbolt WiFi to be very efficient. I now never turn it off unless I am on the car/motorcycle (no need to it to keep searching for a connection). It appears that it saves a bit of battery. And if it isn't, it is faster then the 3G connection which is all I normally get.