E-commerce: where is the best place in a payment stream to ask users to save the payment method?

In a payment process, I need to show an option for the user to save the information of their payment method, to pay faster next time. Only registered users can save this information, therefore, if a user wishes to save, they must sing or create an account. The payment form is a single form, with no steps.

2 options come to mind:
1) After the user enters the payment information, before sending the payment. This will activate a modal window to login or register.
2) After processing the payment, on the success screen.

There are advantages and disadvantages to any of these, but I am curious to know what you think.