The 3G can come in handle during a power outage if you suddenly need to check something on the net or want a book to occupy your time.
If you leave the Wifi on with the 3G model your battery drain will be significantly more then just the WiFi model. So if you want longer battery life keep WiFi off unless you are using it. If you plan to keep WiFi on all the time you probably want to go with a WiFi only.
The 3G model is a little more heavier then the WiFi version.
My main complaint is you can't turn 3G off even if you are using WiFi it stays on in the background so the battery drains fairly quickly. So keeping WiFi off it last about 3 weeks with normal use before it needs a charge. If you leave WiFi on you might get a week's worth of battery before it needs charging.