Well, if you stay with your fiancé's plan then you don't get unlimited data unless your fiancé moves to the TMO plan.Is that better than changing my fiancé's existing plan to the new plan and adding me to it? We will both want to keep our same numbers.
If he just adds you to the plan, whatever one that ends up being, then no-one gets the Hookup Code.
The Hookup Code is 20% off the total bill - for life.
If you port in you can use the Hookup Code and you keep your number. Add him to your account and he still keeps his number and now the code applies to the total bill.
But don't take my word for it. Verify with a CSR from T-Mob.