3G drains the battery like you wouldn't believe. I can go a full 24+ hours on a single charge, but 3G trims that down to somewhere around 6. For me, anyway.
Wifi though? I can go probably three times the length of time I get with 3G. I haven't used wifi at any one time for that long, but measuring up the rate of battery drain from one versus the other, I'd be willing to bet wifi will hit 3x that mark.
TL;DR Wifi. Use Wifi unless you have to use 3G.