switching from ATT to Tmobile

Few questions...

1. I am switching from ATT to T Mobile ... my 128 6 plus is now unlocked . What's the best place to sell it?
2. Signed up for T Mobile already but want to transfer my phone number... Should i order the 6s Plus with the T Mobile Sim card or sim free?
3. I am using a gift card to order my phones and my dads through the app. I will have a remaining balance of a few hundred, will it let me use a credit card? is there a way to enter the gift card prior or do i have to do it on the spot?. Can i ship one of them to a different address?
4. Anyone have experience with T Mobile Service in Los Angeles? is the lte pretty good?
5. I am debating switching to the 64gb now cause i mostly only put audio books on my phone and pictures and some video . Don't really put music unless its a mixtape.
6. How hard is it to get approved for Apples finance plan?

 

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