If you are an existing Verizon customer you can transfer the phone to her and go to tiered data on HER line. You would keep unlimited on yours by buying outright.
I am not an AT&T customer, but I'm pretty sure they went to tiered awhile ago. They just recently announced the same Family type plan where everyone draws from the same bucket of data. The main difference that I know between them is new customers with Verizon can ONLY choose the Share Everything plan. AT&T allows new customers to choose tiered or Share Everything.
I hate the shared data plans that are available, I haven't seen one yet that seems in any way beneficial to me. I just hope AT&T doesn't force existing customers off unlimited plans to qualify for subsidized pricing, my iP4 is showing its age and I really don't want to have to make the choice between subsidized phone or keeping my unlimited plan.