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 two main paths—Medicare Advantage (Part C) and Medicare Supplement plans (Medigap)—sound similar on a brochure but behave nothing alike when a hospital bill lands.
Medicare Advantage replaces Original Medicare with a private insurance plan, usually bundling drug coverage and extras like dental or vision.
Medigap doesn't replace anything—it sits alongside Original Medicare and picks up much of what traditional Medicare leaves behind, including that 20% coinsurance with no annual cap.
That uncapped 20% is the detail people miss.
Original Medicare alone has no out-of-pocket maximum, which is exactly why Medigap exists.
A serious illness or a few weeks in a skilled nursing facility can run into the tens of thousands if you're relying on Original Medicare by itself.
These plans do cap your annual out-of-pocket spending—often around $8,000 to $9,000 in-network for 2024, though some run higher.
You trade predictable premiums for the risk of a big bill in a bad health year.
Many Advantage plans also carry $0 premiums, which is why they've grown to cover more than half of all eligible Medicare beneficiaries.
The catch is networks and prior authorization.
Advantage plans typically require you to stay in-network and get approval before certain procedures.
Medigap lets you see any provider nationwide who accepts Medicare, no referrals, no in-network maze.
For snowbirds or anyone who travels, that flexibility alone can decide the choice.
Medigap premiums rise with age and inflation, and you pay them every month on top of your Part B premium.
Advantage premiums can be near zero, but copays and coinsurance add up as you use care.
A healthy 65-year-old may come out ahead on Advantage; someone managing chronic conditions often does better with Medigap's predictability.
One rule trips up a lot of people: you get a one-time Medigap open enrollment window that starts when you're 65 and enrolled in Part B.
During that window, insurers generally can't deny you or charge more because of health history.
Miss it, and you may face medical underwriting—or get turned down entirely.
You can drop a Medigap plan and join Advantage during annual enrollment, but getting back into Medigap later isn't guaranteed in most states.
That asymmetry is the single biggest argument for thinking hard before leaving Original Medicare.
If you go the Medigap route, you'll need a standalone Part D plan, which adds another premium but gives you flexibility to compare drug formularies each year.
Add up your realistic yearly costs under both, factoring in premiums, copays, and the worst-case scenario—not just the best one.
Check whether your doctors and hospitals are in the Advantage network.
And be honest about your health trajectory, because the plan that's cheapest today isn't always cheapest over a decade.
The takeaway: Advantage can be a genuine money-saver for healthy, budget-focused retirees who stay in-network.
But Medigap buys something you can't put a price on—predictability and access when you're sickest.
Final Thoughts
Run the numbers for your own situation, talk to a licensed counselor through your State Health Insurance Assistance Program, and don't let a $0 premium headline make the decision for you.