Unlimited data for iPhone and iPad devices is expensive. Paying $30 per device per month adds up. With the new Tethering option, you get a $45 monthly plan on your iPhone, with the ability to share the plan with other devices, like your iPad. Assuming you don't go over the 4GB data cap, monthly tethering is a better option. By looking at your personal data usage of iPhone, iPad and computer data usage, you can find a model that works for your needs, avoiding unnecessary charges for data plans you will never use.