Medicare's annual enrollment window closes December 7, and the choice between Medicare Advantage and a Medicare Supplement plan is the single most expensive decision many retirees will make this year.
In practice, they can differ by $5,000 or more annually, depending on your health and where you live.
Medicare Advantage (Part C) is run by private insurers under federal contract.
Plans often advertise $0 monthly premiums and extras like dental, vision, and gym memberships.
The trade-off: networks, prior authorizations, and copays that can climb fast if you get sick.
A 2024 KFF analysis found that nearly half of Medicare Advantage enrollees with significant health needs spent more than $2,000 out of pocket in a year.
Medicare Supplement plans, also called Medigap, work differently.
You pay a monthly premium to a private insurer, and the plan picks up most of what Original Medicare doesn't, including the 20% coinsurance with no annual cap.
There are no networks, so you can see any provider who accepts Medicare.
The catch is cost: Medigap premiums commonly run $100 to $250 per month, and they rise with age in most states.
The math tilts based on how often you use care.
A healthy 65-year-old who rarely sees a doctor may come out ahead with a low-premium Advantage plan.
Someone managing diabetes, heart disease, or cancer often hits Advantage plan copay limits quickly, while Medigap keeps costs predictable.
One detail trips up a lot of people: Medigap has a one-time open enrollment window that starts when you first sign up for Part B.
During those six months, insurers must sell you any policy they offer, regardless of health history.
Miss it, and you can be denied or charged more in most states.
Advantage plans, by contrast, let you switch annually during open enrollment but come with the network and prior authorization rules.
If you're already in an Advantage plan and considering a switch, ask your doctors two questions: Are you in my plan's network, and do you require prior authorization for my treatments?
A "no" on the first or a "yes" on the second is worth pricing out a Medigap alternative, even at a higher monthly premium.
Also check whether your state has a "birthday rule" or annual guaranteed-issue window.
A handful of states, including New York and Connecticut, let you switch to Medigap year-round regardless of health.
That safety net doesn't exist in most of the country.
Both paths now run through Part D, but formularies and tiers vary wildly.
Plug your actual medication list into Medicare's Plan Finder at medicare.gov rather than trusting a brochure.
If you do nothing, your current coverage generally renews, but a plan you picked years ago may have quietly changed its network, copays, or drug tiers.
Fifteen minutes on the Plan Finder can surface a difference of hundreds of dollars a month.
Our take: treat this like any other household bill, not a set-and-forget decision.
The cheapest premium is rarely the cheapest plan once you actually use it.
Final Thoughts
Run your own numbers, call your doctors, and lock in a choice before December 7.