I have absolutely no supporting information, but I would hazard to say that 90% of the time, Wi-fi will be faster than 3G.
i will backup this statement. cable internet, and even verizon fios is pretty common nowdays, at least in my area. it's also pretty common for businesses to have a wifi router and have it unprotected. i was amazed at the amount of places i could connect to with wifi.
my wifi at home i have seen max out at 18mbps, the highest i have ever tested over 3G on a good day was 2.15mbps. now if we could get 2.15mbps over 3G everywhere that would be decent, but it's not that common, at least in my neck of the woods. also 3G had pretty bad latency, usually upwards of 100+ms. my cable connection is home is about 8-20ms.
also, i have seen no evidence that wifi eats battery. i have actually seen evidence saying that wifi will save battery. look at the specs on the Nexus 1. they claim longer battery life when the phone is using wifi.