Ask HN| being a department in big company or being small company?

OK, we are a team developing website often with WordPress, and the work comes from some big company (they redirect the clients to us), recently they offer us to be a department in their company (that came after our team decided to make small company for his own and we ask them to put our name in the footer in the website we will develop)-

so what is the best make small company for our own , or to be department in big company?