Android - Which consumes less battery: Wi-Fi or 2G data?

I already posted this in another answer. As it where different details that time (more a general "what consumes most"), here some details from a reference Motorola Droid. Data taken from a German article named Energiesparplan (Heise is a very famous technology publisher in Germany; Google Translate Version here):

┌────────────┬──────────┬────────────┬──────────┐
|  Standard  |  Upload  |  Download  | Standby  |
├────────────┼──────────┼────────────┼──────────┤
|  3G        |  1410 mW |   1349 mW  | 18,3 mW  |
├────────────┼──────────┼────────────┼──────────┤
|  2G        |  1179 mW |    853 mW  | 11,6 mW  |
├────────────┼──────────┼────────────┼──────────┤
|  Wifi      |   479 mW |   1158 mW  |  7,8 mW  |
└────────────┴──────────┴────────────┴──────────┘

Guess this definitly answers the question. Though the values may vary between devices, relations should roughly match.

The generic perception is that networking via Wi-Fi cause less drain on your device's battery than connecting with 3G.

During the data transfer, if you use both connections to perform the same download, you will find that Wi-Fi is more efficient since it transfers stuff faster than 3G, thus leading to less consumption of your battery.

^{Rare exceptions may prove otherwise, but 3G faster and with stronger signal than Wi-Fi is a grandpa's fairytale ;)}

One important thing to account for, is the network coverage:

If you stay a large amount of time within a limit coverage area, either for 3G or WiFi, disable the one who's suffering from low coverage. Your device will constantly be searching for available network, and that's just energy wasted on fruitless searches.

^{Being this usually the cause for complain on the battery life.}

2G consumes less power than WiFi when it comes to standard usage (surfing, small downloads etc). The problem with WiFi: It constantly scans for Hotspots even when you are connected to one and not using it.

When it comes to big downloads etc, WiFi is more efficient. Reason: Due to slow speed, 2G will continue to drain battery for 2 hours when WiFi quits downloading within 5 minutes.