Every fall, millions of Americans on Medicare face the same fork in the road, and the wrong turn can quietly drain thousands from a retirement account.
The decision comes down to two very different products: Medicare Advantage (Part C) and Medicare Supplement plans, also called Medigap.
They behave nothing alike when a hospital bill shows up.
Medicare Advantage is the all-in-one option.
Private insurers like UnitedHealthcare, Humana, and Aetna bundle your Part A and Part B benefits, usually add drug coverage, and often throw in dental, vision, and gym memberships.
The pitch is appealing because many plans carry a $0 monthly premium beyond the standard Part B charge.
Advantage plans run on networks, copays, and annual out-of-pocket maximums that can reach $8,850 for in-network care in 2025, according to Medicare.gov.
You can also be hit with prior authorizations, and a 2023 federal report found some insurers denied requests that should have been approved.
You keep original Medicare and buy a supplemental policy from a private insurer that covers most of the gaps, like the 20% coinsurance on Part B services.
You see any provider who accepts Medicare, anywhere in the country.
Medigap premiums vary widely by state, age, and plan letter, often running $100 to $250 or more per month.
Unlike Advantage, you also need a separate Part D drug plan, which adds another premium.
Here's the detail that trips people up: Medigap is easiest to buy during your six-month Medigap Open Enrollment Period, which starts the month you're 65 and enrolled in Part B.
After that window, in most states, insurers can charge you more or reject you based on health history.
Someone who picks a cheap Advantage plan at 65 and develops diabetes or heart trouble at 72 may find that switching to Medigap is prohibitively expensive or simply impossible.
In a handful of states, including New York and Connecticut, rules are more forgiving, but most Americans don't live there.
A healthy Advantage enrollee might spend very little.
Someone who needs surgery, cancer treatment, or frequent specialist visits can blow past several thousand dollars in copays before the out-of-pocket cap kicks in.
A Medigap enrollee pays higher premiums every month but faces far smaller bills when care actually happens.
Advantage can work well for people who are comfortable with networks, rarely travel, and want low fixed costs.
Medigap tends to suit people who want predictability, see specialists, or split time between states.
Before you decide, check three things: whether your doctors are in the Advantage plan's network, what the plan's out-of-pocket maximum really is, and whether you could pass medical underwriting later if you wanted to switch.
Free counseling is available through your State Health Insurance Assistance Program, or SHIP.
Our take: the cheapest premium is not the same as the cheapest care.
If you can afford Medigap early and value flexibility, locking it in at 65 is often the safer long game, because the door can close behind you.
Final Thoughts
Run your own numbers with a SHIP counselor before Open Enrollment ends, not after the bills arrive.