since you have excellent 3G, your handsets radio will use less power than if you were in a fringe area.
wifi also behaves the same way.
really ,, at your home with excellent coverage on both ends.. its a wash.. if you were in a fringe 3G area, id use wifi.. Wifi would use less energy than 3G overall. im no cellular expert though, however i think 3G uses more energy than wifi. soooo... stick with wifi...
clear as mud? ;0)