Every fall, millions of Americans on Medicare face the same decision, and most get it wrong by guessing.
The two main paths—Medicare Advantage and Medicare Supplement plans—look similar on a brochure but behave nothing alike when a hospital bill lands.
Picking the wrong one can mean the difference between a predictable monthly cost and a surprise bill that runs into the thousands.
Original Medicare (Parts A and B) pays roughly 80% of covered costs, leaving you on the hook for the other 20% with no annual cap.
A Medicare Supplement plan, often called Medigap, picks up most or all of that gap.
You pay a higher monthly premium, but your out-of-pocket exposure stays small and predictable.
Medicare Advantage works the opposite way.
These are private plans that replace Original Medicare, usually bundle in drug coverage, and often advertise $0 premiums.
The trade-off is networks, prior authorizations, and copays that can climb.
Advantage plans do cap annual out-of-pocket spending, but those caps can sit in the thousands—and care outside the network may not count the same way.
The premium math is where people get tripped up.
A Medigap Plan G might run $120 to $200 a month depending on your age and state, on top of the standard Part B premium.
An Advantage plan might cost $0 in premium.
That gap looks like easy savings—until a specialist visit, a scan, or a short hospital stay triggers cost-sharing you didn't budget for.
There's a deadline trap that catches people every year.
When you first enroll in Medicare at 65, you get a one-time Medigap open enrollment window that lasts six months.
During that window, insurers can't charge you more or deny you coverage because of health conditions.
Miss it, and in most states you can be underwritten—meaning a past diagnosis could block you or spike your rate.
Switching from Advantage back to Medigap later is the part that blindsides people.
If you've had an Advantage plan for years and develop a health issue, moving to a Supplement plan outside your guaranteed window can be difficult or expensive in most states.
A handful of states offer year-round protections, but most don't.
That asymmetry is why advisors often tell healthier people to think hard before trading guaranteed access for a low premium.
Neither option is universally better—it depends on your health, your doctors, your travel habits, and how much uncertainty you can stomach.
Advantage plans can be a strong fit for people who want low fixed costs and are comfortable staying in-network.
Medigap suits people who want to see any provider who accepts Medicare and hate surprise bills.
The practical move is to check whether your doctors and hospitals are in a plan's network before you commit, and to read the annual out-of-pocket maximum, not just the premium.
A $0 premium with a $6,000 cap is a very different product than it appears on the flyer.
The real lesson here is that the cheapest-looking plan is often the most expensive one in a bad year, and the deadline to lock in your best protection is easy to miss.
Final Thoughts
Run your own numbers with your own medications and doctors rather than trusting the headline price.