Which is the better doctor, a DM cardiology or a DNB?

There is no simple answer to this question. It depends on the specific medical conditions and needs of the individual patient. Generally speaking, most patients would be better served by a doctor who has completed a residency in internal medicine and then completed additional training in cardiology (i.e., a DM). However, there are some excellent DNB cardiologists out there as well. It's ultimately up to the patient to do their own research and decide which doctor is best for them.