(I can't post here the link to the app but you can search for the app of for "Kapex" on Google Play)
Auto WiFi 3G will check every defined time for known wifi connection by turning on wifi, if it finds a known wifi it will turn off 3G (Also the antenna).
In case you are already connected to wifi and you loose the connection it will turn on 3G and turns off WiFi (Also the antenna).
WiFi in most cases is faster and consumes less battery.
What do u think about the app?