A refund goes back to the card or account it came from; a credit stays on your account and is applied to your next invoice.
Refunds
Approved refunds are returned to the original payment method within five working days. The refunded invoice keeps a record of the transaction, so your statement and ours agree.
Account credit
Credit comes from a downgrade, a goodwill adjustment or funds you added yourself with Add Funds. It is applied automatically the next time an invoice is generated, and any remainder stays on the account. Your balance is shown on the dashboard and on the Add Funds page.
Which one you get
Money you paid for something that was not delivered is refunded. Value you are owed for a change of plan is credited. If you would rather have a credit refunded, open a ticket with the Billing department.