I own and use both and the difference is not worth worrying about. If my iPad only did 6 hours it would be way more than most would ever use before being able to recharge it. The battery life is outstanding on both.
3G is SLOW in comparison to using WiFi, if anything, it will burn up more batteries just waiting on 3G in comparison to WiFi. BUT when WiFi is not available, it is worth having 3G. I was scared people would hog and clog the network and just leave it on 3G but once you have used WiFi you will use it for the speed!