I wouldn't be able to comment on the value of credentialing an MPH versus certificate.
However, with the current administration's goal of repealing the ACA, there's a markedly increased need for OMS as a profession to encourage novel and robust research in public health to supplement AAOMS and OMSPAC engagement in the public debate about reimbursement for OMS procedures. An article in press from JOMS by Dodson discusses how insurers like Aetna frequently cite prominent leaders and research articles in shaping its coverage policy. Public health research such as a recent paper published by Salomon and Schlieve proves the value of OMS procedures by revealing the spike in cost and number of ED visits for odontogenic infections after the state of Illinois cut back on Medicaid dental benefits.
But the problem is that there's a noticeable dearth in number of public health research produced among OMS programs. That same paper cites the possible reasons as "lack of recognition of its [public health research] importance, lack of expertise within the field, and lack of incentives to conduct such research." The second citation regarding lack of expertise is probably where that MPH or certificate could help add more value to your education.
Already, the reimbursement rates for facial trauma and orthognathic surgery are abismal.