Should I purchase one thru ATT or go to a big box store and buy it. Are there additional fees to pay if I get it thru the box store that are built into the ATT price?

Advantages, I'm not the least interested in a Mini as this will be used for client presentations.

If they are bluetooth capable I will give up my iPhone and transfer everything to the iPad. Including my personal hotspot (no jokes please) do I sell (or can I ) an I-3 and an I-4 ? Or can they be used as a trade-in at ATT?