While evaluating the Medicare choices for a relative two years ago I looked at Medicare Advantage plans. In my relative's area in Northern California, standard Medicare lists around 2,800 doctors in a 25 mile radius while not stating their specialities..
Most advantage plans offered less than 200 hundred local doctors, while not stating their speciality either. Looking at the physicians most of them appeared to be young (read inexperienced) and I didn't come across any who were board certified in family medicine. My relative's current primary care doctor - who is board certified - was not on any of the numerous plans that I checked.
Needless to say, I recommended staying far away from Advantage plans. There are many TV adverts for these plans around December (usually around two minutes long and in the middle of the day) and a huge number of mail offers, so they must be very profitable for the health insurance industry. That, by itself, is a bit worrying as they are less expensive and generally offer additional services (such as dental, routine vision care and free transportation to doctor visits.)
My relative chose standard Medicare and has been happy with the care she has received. She hasn't had to change any of her doctors (family medicine, rheumatologist, neurologist, podiatrist and probably others I'm unaware of.)