I was struggling with this myself, but I'm going with the WiFi version for a few reasons.
I mostly plan to use the iPad for reading technical .pdf's, so I can get away without 3G there.
I'm really not sure how often I'd want to take it out of the house. It's more like lugging a laptop around than slipping an iPhone into my pocket, so I doubt I'd actually bother taking it with me as much as I think I will when I watch the marketing videos. I love having my MB for trips, but I almost never take it out for quick errands (where 3g would be needed).
The data plan is pretty good, but not THAT great. I'm really happy that Apple negotiated a contract-free plan with AT&T, and when comparing it to Verison's offerings it looks pretty amazing. I would get the unlimited plan even though I probably would never even hit the $250MB cap simply because it's worth the extra expense to have the peace of mind not to worry about keeping track of my data usage. But then you're paying $30/month for less data usage than the iPhone--which I'm sure will pull more data over the course of a month due to being with me at all times.
Lastly, I expect AT&T to allow tethering for the iPhone sometime in the near future (I hope). With this possibility, and my usage patterns, tethering an iPad to my iPhone on the infrequent occasions where it's necessary is probably the wisest, most cost-effective strategy.
The biggest reason in favor of the 3g for me is that it's a permanent decision that I might later come to regret if my situation changes down the road. I have this mental image of myself having to by a second iPad to replace my WiFi model because I was too cheap to spend the extra $130 (pennywise, pound foolish).
Ultimately, I think I'm going to go with the WiFi because it's the best fit for my needs, and if things change I can always give my parents the WiFi model and pick up the 2.0 version or the pro model whenever that comes out with the 3g support then. Hopefully this was helpful to someone other than myself.