Where to buy new phone?

[musiclawyer]

I have an old iPhone 4s. ATT Paid off /out of contract. I want a new SE--price cash I will pay. I wish to leave ATT for TMobile because I'm moving and TMobile is better option after the move and I go into Canada often ...

Would you buy at the TMobile corporate store or at the Apple Store? They both have the same small offset in price for turning in old phone.

At first I figured the TMobile corporate store would be best option , but now I wonder if TMobile can help me wipe the old phone properly before I turn it in ( in case I have issues )

Thanks for any advice.