Kindle 3 - should I get WiFi, or WiFi+3G?

I understand the Kindle 3 has two versions, one with WiFi, and one wtih WiFi plus 3G. What exactly is the benefit of having 3G on your Kindle for all of that extra money? I already have a smart phone with 3G…is there something I’m missing, or is the only benefit that I’d be able to download books in places with no wireless?

Which one do you recommend? Not getting the nook or an Ipad, sorry. Kindle-related responses only, please.