It depends on what you want to have.
As far as I know the ISPC billing module is just a simple solution for invoicing some services.
But “WHMCS is an all-in-one client management, billing & support solution for online businesses. Handling everything from signup to termination, WHMCS is a powerful business automation tool that puts you firmly in control”.
So you have to choose: ISPC billing module or WHMCS and ISPC Server Module.