Daniel Indiviglio takes a look at how you can get data for your iPad. In the end he doesn’t declare a winner. Here is the answer though:
If price is the biggest factor for you then you get more bang for your buck if you go with Verizon’s $20 1gb a month plan. If you have crap Verizon service in your area then go with AT&T and vice versa.
If money is no object and you get great service from both companies (like we do here in Seattle) then why would you want to have to carry around the MiFi and your iPad? I hate doing it, but I didn’t want to wait for the 3G model (probably will for the 2nd gen version though).