If you're turning 65 or reviewing your coverage right now, you've probably stared at two thick brochures: Medicare Advantage and Medicare Supplement.
And picking the wrong one can swing your yearly health costs by several thousand dollars.
Original Medicare (Parts A and B) covers hospital care and doctor visits, but it leaves gaps — deductibles, the 20% coinsurance on many services, and no cap on what you pay in a year.
Both Advantage and Supplement plans exist to fill those gaps, but they do it in completely different ways.
Medicare Advantage, also called Part C, is run by private insurers like UnitedHealthcare, Humana, and Aetna.
You usually pay a low or $0 monthly premium, and many plans throw in extras like dental, vision, hearing, and gym memberships.
The trade-off: you're locked into the plan's network, you may need referrals, and you can face prior authorizations.
If you get care outside the network, you often pay the full bill.
Medicare Supplement, often called Medigap, works differently.
You keep Original Medicare and add a private policy that pays most of what Medicare doesn't.
That means you can see any doctor in the country who accepts Medicare — no networks, no referrals.
Depending on your state, age, and plan letter, expect anywhere from roughly $100 to $250 or more per month, according to industry pricing data.
Plan G is currently the most popular choice for new enrollees because it covers nearly everything except the Part B deductible.
If you're healthy and rarely see doctors, a $0-premium Advantage plan can look like a steal.
But one bad year — a surgery, a cancer diagnosis, a specialist who's out of network — can flip that math.
Advantage plans do cap your annual out-of-pocket costs, but those caps can run several thousand dollars.
Medigap plans typically leave you with almost no surprise bills.
When you first enroll in Medicare at 65, you have a one-time Medigap open enrollment window of six months.
During that window, insurers can't charge you more or deny you coverage because of health problems.
Miss it, and in most states you can be underwritten — meaning a diabetes diagnosis or a past heart attack could get you rejected or charged more.
You can switch back to Original Medicare during certain enrollment periods, but getting a Medigap policy after that first window often requires passing medical underwriting.
In other words, the cheap plan you pick at 65 could be hard to leave at 75.
A few practical moves: check whether your doctors accept the specific Advantage plan, not just "Medicare." Add up your prescriptions on each plan's formulary.
And if you travel a lot or split time between states, Medigap's nationwide flexibility tends to matter more than the lower premium.
One more note for 2025: the Part D drug benefit changed.
Out-of-pocket prescription costs are now capped at $2,000 per year, which softens one of the old arguments against Advantage plans — but it also changes the math for everyone, so run your own numbers instead of trusting last year's advice.
The honest take: there's no universal winner, only a winner for your health history, your doctors, and your budget.
Final Thoughts
Spend an hour with the Medicare Plan Finder tool and a calculator before you click enroll.