I'm super torn about this, but for a reason I've not seen mentioned here.
I would love to save the cost on the radio and data charges. As a related aside, my intent on the data would be to literally turn the data plan on and off on the fly as needed. If AT&T pro-rates this feature as they do all iPhone add-ons, this would be a relatively cheap way to get 3G data only when you absolutely needed it. You'd still have to pay the $130 premium for the radio of course.
If I went with wifi-only I could of course JB and use MyWi or similar as has been pointed out. HOWEVER, for me there's one big issue with this: international travel. SJ mentioned that international data deals would be announced soon. Hopefully this means prior to shipping either model.
The issue is that AT&T's international data roaming rates for the iPhone are f*#$ing insane. Like prohibitively expensive such that when I travel I turn off data roaming and use wi-fi only. This works "okay" but there are definitely times and places where there's no wifi, and so I basically can't use my iPhone.
So to have to use such a plan with a JB'd iPhone in order to surf on the iPad in such places would cost a fortune, at least until we hear what the international data deals look in terms of cost, contract requirement or not, etc.
All that said, wifi only means 30 days sooner
But I have to think long term here as I intend to use my iPad as my primary computer...
TM