Nearly everything written about this choice describes symptoms. The cause is a single structural fact, and once it is clear the rest of the differences stop being a list to memorise and become consequences of one thing: who is the payer.
The cause
A supplement policy leaves Original Medicare in place. The federal programme remains the payer, the policy pays part of what the programme does not, and the standardised letter on the policy decides which parts. A Medicare Advantage plan replaces that arrangement: Medicare pays a private insurer to take on the whole risk, and the insurer becomes the payer, with its own card, its own rules and its own contracts with providers.
The four consequences
Networks exist under one and not the other, because a private payer contracts providers and a national programme does not. Referrals likewise. The shape of the cost differs, one charging more monthly and less at the point of care, the other the reverse up to an annual limit. And reversibility differs, because moving back from a plan to a supplement can require medical underwriting depending on the state and the timing, which is the consequence people notice last and regret most.
The measurement this site adds
Which arrangement is ordinary is not a national fact. Every state page here carries the split from CMS's own enrollment file, and it runs from most of a state having moved to a private plan to almost none of it. That is not an argument for either choice. It is the context that is missing from every article on this subject, and it is the only thing on this page that is a measurement rather than a description.
Questions people ask about medicare supplement vs advantage
Which is cheaper?
They are not comparable on a monthly figure, because one charges more monthly and less when care happens. This site quotes no premiums at all.
Can I switch later?
Moving into a plan is straightforward in a window. Moving back out and buying a supplement can require medical underwriting, depending on state and timing.
Does a supplement have a network?
No, because Original Medicare remains the payer. Network and referral rules belong to the private plan arrangement.
Which does this site recommend?
Neither, ever. It publishes what people in each state hold and points at free, non-commissioned counselling.