How does power usage for data over WiFi compare to data over 3G? (Ignore LTE for now)
I never really did a precise comparative test but it is clear that wifi uses way less power than the 3G data connection. So I would recommend to use wifi whenever possible if the goal is to extend battery life.

The only exception I would see is if you often use the phone but for a very short moment at a time, the delay it takes for the wifi to connect can be a hassle. In this case only I would deactivate the wifi.