First of all, we launched our open source billing repository months before they did. In fact, they even reached out to us for inspiration. This could be related to the fact that our team has prior experience building billing engines, so we have a better understanding of the space.
From a product perspective, our coverage is more extensive. We provide more advanced coupons, usage-based components and billing use-cases. For example, we offer credit notes and refunds, small examples that are not listed in their API. Our product covers more billing edge-cases and deeper features.
Finally, we have customers in production. Our top user is invoicing 5M customers a month, which might not be the case for them.
To conclude, I would assert that we are more advanced.