Every fall, millions of Americans on Medicare make the same costly mistake without realizing it.
They let an insurance ad or a pushy call center pick their coverage, then spend the next year discovering what isn't covered.
The choice at the center of it all: Medicare Advantage or a Medigap supplement.
And the gap in what you pay out of pocket can run into the thousands.
Medicare Advantage, often called Part C, is run by private insurers.
You typically pay a low or $0 monthly premium, and many plans bundle in dental, vision, hearing and prescription drugs.
That low sticker price is why roughly half of eligible beneficiaries now choose it.
The catch is what happens when you actually get sick.
Advantage plans use networks, prior authorizations and copays.
A hospital stay, a chemo regimen or a skilled nursing stay can trigger daily charges that add up fast.
Federal rules cap in-network out-of-pocket spending, but that ceiling can still reach $8,000 or more in a single year.
A Medigap supplement works the opposite way.
You keep original Medicare and add a private policy that picks up most of the leftover costs — deductibles, coinsurance, the 20% Medicare doesn't pay.
Premiums run higher, often $100 to $250 a month depending on your state and age.
In exchange, your exposure is small and predictable.
There's a timing trap most people never hear about.
If you enroll in a Medigap plan during your initial six-month window, insurers must sell you one regardless of your health history.
Miss that window, and in most states they can reject you or charge more for a pre-existing condition like diabetes, cancer or heart disease.
That single deadline is why financial planners tell healthy 65-year-olds to think hard before grabbing the cheapest Advantage plan.
Advantage plans aren't automatically the wrong call.
If you're healthy, take few prescriptions, and want low fixed costs, they can work well.
But the marketing is relentless, and some agents earn higher commissions steering seniors one direction.
Check whether your doctors and hospitals are in the network.
List every prescription and price it through the plan's formulary.
Read the plan's maximum out-of-pocket limit, not just the premium.
And if you're leaning toward a supplement, confirm your state's rules before your guaranteed-enrollment window closes.
The difference between these two paths isn't a small budgeting detail.
It's the difference between a capped, predictable year and a bill that arrives right when you can least afford it.
My take: the cheapest premium is rarely the cheapest coverage.
Final Thoughts
Spend an hour with the official Medicare Plan Finder before you decide, and treat that six-month Medigap window like the deadline it is — because for most people, it never reopens.