Initially I ordered both the 3G and wifi as I didnt know which one to get. Over the weeks before it was released I decided on the wifi version because I couldnt see the purpose of the 3G. I have a data plan on my iphone so if I want to use the internet on the move I use that, I also have a dongle for internet on the move. I also can't imagine wanting to buy books whilst im out.
It depends on whether you think you will need/use/want the 3G.