I have the 3G model and I can say from what I have read the WiFi has better battery life if you plan to leave the wireless on. If you turn it off they are about the same. I can say 3G comes in handy specially during a internet or power outage! We had one the other day and my books library is on my Dropbox so I was able to grab a book I wanted to read with no problems.
If you leave WiFi on with the 3G model you might get a week charge at the most depending on usage. Reason being you can't turn 3G off even when you are in WiFi mode.
Something I didn't know about till I got it. Also 3G is slight heavier then the non 3G version.
Oh Indexing really drains your battery so plug the Kindle in when you add alot of books. It takes like 10 regular size books per 30 minutes to index on mine.