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.