Wifi uses less battery (assuming you are in a decent area and the phone isn't constantly trying to get a signal from the router). It may also be a little faster, but nothing earth shattering usually.
Another advantage of using Wifi over 3G is if your in a area with no 3G coverage but does have Wifi coverage.
I'm not sure but maybe if coupled with google voice, if you were overseas and had no cdma 3G available, you might be able to actually make calls through the Wifi?