If you're turning 65 or reviewing your coverage right now, you're staring down one of the most expensive decisions in retirement.
Medicare Advantage and Medicare Supplement plans both fill the gaps in original Medicare, but they work in completely opposite ways.
Pick the wrong one for your health situation and your wallet, and you could be stuck with it until the next enrollment window.
Medicare Advantage, also called Part C, is the all-in-one option.
Private insurers bundle hospital, medical, and usually drug coverage into one plan, and many charge a $0 monthly premium on top of your Part B payment.
The trade-off hides in the fine print: copays, coinsurance, prior authorizations, and provider networks.
A 2023 KFF analysis found that a hospital stay can cost Advantage enrollees hundreds or thousands in cost-sharing that a supplement would have covered entirely.
Medicare Supplement plans, often called Medigap, do the opposite.
You keep original Medicare and add a private policy that picks up most of what Medicare doesn't pay, including deductibles and coinsurance.
You can see any provider nationwide who accepts Medicare, and there are no networks or referrals.
The catch is the monthly premium, which commonly runs $100 to $250 or more depending on your age, gender, and state.
The financial gap between the two paths is real.
Someone who hits a serious diagnosis could face $5,000 to $7,000 or more in out-of-pocket costs in a bad year on an Advantage plan, while a Medigap Plan G holder might cap out around $240 for the year, plus premiums.
For a healthy 65-year-old who rarely sees a doctor, the math can flip the other way, since Medigap premiums keep coming whether you use care or not.
Here's the trap most people miss: Medigap is easiest to buy during your six-month Medigap Open Enrollment Period that starts when you're 65 and enrolled in Part B.
After that window closes, insurers in most states can charge you more or deny you outright based on your health history.
Switch from Advantage to Medigap five years later, after a cancer diagnosis or a heart condition, and you may be locked out.
A handful of states, including New York and Connecticut, offer year-round guaranteed acceptance, but most don't.
Many include dental, vision, hearing, gym memberships, and over-the-counter allowances.
Some have $0 premiums and low copays for routine care.
If you're healthy, take few prescriptions, and don't mind staying in-network, the savings can be meaningful.
But you need to read the plan's drug formulary and provider directory every year, because networks and covered drugs change each January.
List every doctor and every prescription you use, then check them against each plan's current formulary and directory.
Estimate your worst-case year, not your best-case one, including hospital stays and specialist visits.
And if you're leaning toward Medigap, buy it during that first six-month window while you still have guaranteed acceptance.
Medicare's Plan Finder at medicare.gov lets you plug in your drugs and doctors and compare real numbers.
Calling your state's Senior Health Insurance Information Program, or SHIP, gets you free one-on-one counseling with someone who isn't paid by an insurer.
The bottom line: Advantage wins on monthly cost, Medigap wins on predictability and freedom.
Most people shop the premium and ignore the risk, and that's exactly how a $0 plan turns into a $6,000 hospital bill.
Final Thoughts
Run your own worst-case numbers before you click enroll, because in most states, this is one decision you don't get to take back.