Intuit Defines the differences as follows:
The Master Admin is typically the user that has set up the file and is the owner of the file.
The Company Admin has access to all of the features and capabilities of the QBO file and can see everything.
The Master Admin is the only one that can transfer their role to another person.
We have found that if there are two accountant users, and you want to remove one and invite another, it appears as though only the Master Admin can do that. If there are empty accountant user spots, a Company admin can invite them, it appears to be the deletion of one accountant/bookkeeper that can only be done by the Master Admin.
Also, if the client is on our wholesale billing account, where we are paying the monthly subscription fees, we always ensure that we are set up as the Master Admin.