I can say with a fair degree of confidence (haven't done any methodical testing) that 3G eats up more battery than WiFi. On both the iPhone and the Droid, I find that leaving WiFi on gives me better battery life than if I had turned off WiFi and relied on 3G.
The problem is, very few real-world users actually write down any real data on usage... They mostly give their opinions, which may be incorrect.
But, based on what I've read in various places, it would seem to me that WiFi is a bit more efficient, most of the time.
But I don't use WiFi because a couple of my widgets don't work with it. And the difference between 3G and WiFi is pretty slim, at any rate -- otherwise, there'd be no argument about which is more efficient. It'd be very clear that one is better than the other.