There is a middle ground you may not have examined. I do the CODING but send my invoices to the biller. This way I don't have to trust them to extract all the necessary information, and I stay better in touch with the billing: for instance, what must be charted to achieve a level 4. A lot of doctors do level 4 work, but only get paid level 3 because they left out a small piece in their work, the coder didn't catch it, or they are not confident in having qualified for this code.
My billing company handles everything else for 6%. They bill primary, secondary, and tertiary payers, they use their PM software which I can access from my desk, they tell me who needs to go to collections, they handle patient phone calls about their charges, and so on. For 8%, they will do all the coding from your notes as well. This is a percent of COLLECTIONS, so it is in their interested to collect every dollar they can. I know the billers personally, and they handle this for half the doctors in town.