I'd also point out that they are very different pieces of software. As far as I'm aware Invoice Ninja (which I haven't actually used but looks great) is only an invoice generator and client management tool, while FOSSBilling has integrations with server control panels to automate provisioning of services when an order is placed/renewed. Obviously that isn't important if you are using it to sell services that are not web hosting related, but it's a huge difference if you are.
Invoice Ninja is a more mature platform and definitely worth a look if you are selling digital or physical services that do not need auto-provisioning, but it's not really comparable to something like WHMCS/Blesta/FOSSBilling.
