If you are planning to purchase an iPad 2 with 3G you will need to device which carrier to go with. CNN has broken down the cost differences between the two.
As you can see in the chart below, Verizon clearly wins for those looking to use anything more than 4GB per month. However, before deciding on a carrier you may want to do a little speed comparison in your area. Wired notes that AT&T "on average scored significantly better in speed tests: 62 percent faster for downloads and 38 percent faster for uploads."
AT&T grandfathered unlimited data plan! AT&T wins! Verizon may have more coverage but AT&T is waaaay faster! Guess it depends where you live but Verizon has always been slow in my area with full bars. My opinion....