Wifi is designed to be fast and consume little power. This is especially true if you have a fairly recent phone. The drawback of this is distance or range. You trade the power savings for distance when you use a cellular connection. Therefore, a cellular connection does take up quite some power. This can be easily verified by turning on airplane mode and then turning on wifi. Take note of how long the battery lasts.
In other words, a poor 3G connection will absolutely destroy your battery life. I wouldn't worry so much about keeping Wifi on all the time and letting it do its thing in the background since it sips very little power on standby. So to answer your question, disable 3G when you're in an area with poor connection. Keep wifi on all the time.