Wifi uses much less battery than 3G. All apps will use Wifi over 3G if both are on and connected. Turning off 3G will save some battery, but if you are in a decent signal area, there shouldn't be much difference.
Won't make any difference really because like it was said, if you are on wifi anything that uses data is going to go over wifi so no data will move over 3G. Your 3G radio will still be on for cellular calls even if you turn off data. If you just turned off 3G then you could save some battery life making calls on EDGE.