I just ordered a 13 mini with 512GB.  My payments would be the same with whatever memory level I got as the monthly credit just increased as I increased the memory.
Question on my trade-in: I was given $450 bill credit for the trade in plus the monthly credits.  If instead of sending my phone back, I sold it elsewhere, would I just lose the $450 bill credit and keep the monthly credits?  The way things are worded on the paperwork, it sounds like I really only am getting $450 for my 1 year old 12 Pro and the bill credits are just an upgrade promotion from T-mobile.  I’m thinking maybe I can come out even further ahead if I just sell my 12 Pro on Craigslist or something.  Anyone have any experience with this?