This thread suggests that you turn your WiFi off when you can not use it. It uses some juice if it's on, and if it is doing nothing, that is a waste. When you can use it, it is a batter saver. 3G has to work pretty hard moving large amounts of data, and it is slow, which means it takes longer doing it.
I personally leave my wifi on unless I'm getting low on battery and am away from a charger. At that point I turn off everything I can. It's my only phone, it really does have to be on all the time.