How does power usage for data over WiFi compare to data over 3G? (Ignore LTE for now) If you have unlimited data over 3G so that cost is no option, which is more power efficient? 

Using the WiFi or 3G connection for data usage?

Extending battery life being the goal
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.