Remember the "unlimited" plan is really 5 GB, not unlimited. I think it'll be easy to hit 5 GB a month with an iPad. But I like that you can do the lower plan first, and then upgrade to the larger plan by paying the difference.
I'll use WiFi as much as possible; my Verizon 3G is true unlimited (I signed up before everybody quietly instituted the 5 GB limit), so I'll use that as much as I can, via sharing through my Mac. I would love a MiFi, but I don't think it has an external antenna jack. If it did, I would get one. I use an external antenna in my trailer (I live in an RV) and it makes a huge difference in performance.
Rob