What changes before the month starts?
With prepaid service, payment generally funds a future service period. With postpaid, charges are billed under an ongoing account arrangement. Either can involve automatic payments. A recurring debit does not automatically make a plan postpaid.
The practical question is what happens when payment fails or usage changes. Check renewal timing, allowance exhaustion and any available account controls. For a number you need to retain, an expired prepaid service period deserves attention even if the phone is used infrequently.
What does not follow from the label?
Prepaid does not necessarily mean a basic phone, and postpaid does not guarantee unrestricted data. Device purchases and financing can be separate from either service model. A prepaid customer can still owe a lender for a handset.
Compare supported devices, priority, hotspot, roaming and help channels explicitly. A family account also needs a clear ownership arrangement. Payment timing alone does not tell you whether a line can be moved without changing benefits for others.
Make the comparison fair
Use an ordinary month of usage and the same locations for both options. Include the effect of a longer prepaid commitment, any promotional period and separate handset obligations. Keep the dated plan documents so that a future bill or renewal can be checked against the terms you chose.