Why do you have to have AT&T for both iPad and iPhone?
Nothing wrong with having AT&T iPhone and Verizon iPad. Better that way IMHO, because you will have more diversity of coverage in low-coverage areas.
With the new data sharing plans it may be a good idea to have both devices from the same carrier to get cheaper rates. AT&T announced back in May that they were considering plans similar to what Verizon just announced.