The BB would work better for her daily work needs, but I know she won't want to give up her iPhone for media use. But the only way she would do it is if she could easily switch back and forth between the two.
The bb data plan will work with the iphone. It would be the swapping out of the Sim that would be a pain. I believe the iPhone 4 uses a micro Sim so you would have to get an adapter to make it a regular Sim for the bb