I don't know much about PHPcoin other then it is free. I can say most either use Modernbill (we don't provide) or ClientExec though. You can always try ClientExec out then cancel it if you don't like it.
I was using phpCoin before and using Clientexec now. Based from experience, Clientexec is more robust and customizable than phpcoin. For the price of $5 / month, I would really recommend ClientExec for your billing system.