I am getting a new iPad mini w/retina display and want to know people's experience with the different carriers. I have Sprint as my cellphone carrier and like them. I am trading in a first generation iPad w/64gb+AT&T cellular (which in 3-4 years I've used a total of 2 months) but still want the option in my new iPad. I live in the DC area and 'everyones' service is spotty in places. Any advice?