Commerce Funds includes all of the basics for implementing an escrow or "store credit" system within Drupal. Funds can be created manually or using an existing payment method. Funds can also be used for payment using the included Commerce Funds Payment module.
Commerce Credits uses a credit based system and, when used in conjunction with Commerce Credits Transaction could be used to build a "digital currency" within a site allowing for rule-based credit disbursement and reversible, user-initiated credit transfers. Credits have an exchange rate and the implementation is totally up to the developer.