Android - 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.

I can't remember where I read it but I read that 3G will use more power when transfering data but wifi will use more when idling. I tend to keep wifi on, but if I'm running low on power I'll turn it off to avoid the power requirements of scanning for new networks. Also as others have posted, 2G will use less power than both unless you're downloading large files!

It depends.

Try with your particular phone. Maybe download a large file and see how much the battery drains, then repeat for the other data type. Factors include how many cell towers are nearby, how many packets your WiFi drops, your specific chipset and radio hardware, software drivers, and the like.