So should you buy a Wi Fi only or a 3G iPad?
My standard answer was that for MOST of us, all we would ever need is the WiFi version. Right? Your notebook does not have constant connectivity so you are already used to having to find WiFi to check email etc. And I think for the majority of users, this is going to be a device that is used mostly "around the house".
So I stick with my contention that WiFi is good enough for most of us.
And now I'll tell you why I am going to use the 3G version with the $15/mo limited plan from AT&T. I learned this weekend that I am going to have the iPad with me at times when I would NOT drag around my notebook. Case in point. I went to the construction site this weekend to meet a subcontractor. I took the iPad as I had pdf's of the drawings on it in case we needed to look at the plans. Of course, the contractor was late (actually never showed) but with the 3G iPad I got to have that hour of waiting be productive time clearing my inbox of the 150 or so emails that had accrued from the day before.
The other "case" for 3G I will mention is that since the iPad is so iPhone like... you just assume you are connected to the internet. It is not a big deal, but when working with WiFi only I found my self at lunch or otherwise out and about "waiting" for something and trying to check-in w/ Facebook or Foursquare or trying to check email and then realizing that I had no WiFi in my current location.
I think it will be a very personal decision, but my current suggestion is to just get the WiFi only if you are pretty sure you will just be using it as a shared media device around the house. Or if you have no issue w/ simply syncing your media before you leave your internet connection. Just like an iPod touch... sync your books and movies and music and you are good with the iPad as portable media device.
If you think you will be using it as a productivity (aka work) device and you are already addicted to the constant connectivity of your iPhone, they spend the extra $130 and get a 3G version. You don't have to turn on the 3G (and associated month to month fees) until you are ready, but at least you will have the option.
Guess to make this decision you must "know thyself" pretty well. And you will need to have a good idea of what your use case is going to be for the iPad. I suggest you get a demo from a current user first as my experience is that I am "using" it for a lot of things I had not planned on originally.
Netflix streaming comes to mind immediately ;-)