I bought the wifi only and regretted it almost immediately. Unless I was at home with wifi, it was almost useless. I'd think, "I'll use my iPad for that...oops, no connection". I'd have to find a McDonald's, Starbucks, or a Barnes & Noble to use wifi, and I found it too limiting. After two weeks, I returned it and bought the 3G. I use it just about everywhere. I went to Chicago recently, and I was looking forward to using the hotel's wifi (I thought that it would be free), but it was expensive. If you've got free wifi everywhere, or if you have a portable wifi hotspot like a Mifi or a jailbroken iphone, then good for you.
I think that the question is more of, "do you have an internet connection on the wifi only model everywhere that you need or want, or not?" I don't. I'd love to not have had to pay the extra money, but until wifi becomes more available for me, 3G is better for me.