Question

Differences between plans and prepaid?

Answer 1

Plans-When applying for a plan you’ll need to submit a credit
          check before they can approve your connection.While
Prepaid-You'll need to provide accepted ID before you can
             activate your activate prepaid service.

Answer 2

plan: provides you with the different service option of the service provider; from the plans they present to you, you will choice your best service option. for example. T-Mobile offer unlimited one plan and unlimited one plus plan. you make your choice.

prepaid: this is your billing option which may include post-paid or prepaid. in prepaid, you have to pay for your plan upfront.