Is there any point at all in turning on WiFi when the 3G speeds are so fast? Seems like it would just drain the battery constantly searching for hotspots etc. With a 5GB plan there is little risk of going over the data limit on 3G.
According to Apple, 3G kills the battery faster then WiFi does. WiFi use is 6 hours while 3G is 5 hours. Think you should swap that around.
If you know that you won't have a WiFi signal it could be efficient to turn off WiFi. It is always scanning for a hotspot, so by turning it off you are stopping the constant scan, which will help battery life. But it wouldn't be efficient to turn it off in order to use 3G when a WiFi hotspot is available.