If you're turning 65 this year, you're about to make one of the most expensive decisions of your retirement — and most people get it wrong.
It comes down to two paths: Medicare Advantage (Part C) or a Medicare Supplement (Medigap) paired with original Medicare.
On paper, Advantage looks like the obvious winner.
Many plans advertise $0 monthly premiums, dental, vision, and gym memberships bundled in.
But the catch is hiding in the fine print.
Medicare Advantage is run by private insurers who get paid a set amount per member.
They control the network, the prior authorizations, and the bills.
A 2023 Kaiser Family Foundation analysis found that nearly 1 in 4 Advantage prior-authorization denials were later overturned on appeal — meaning people were initially told no for care they were entitled to.
You pay a monthly premium — often $100 to $200 or more depending on your state and plan letter — but in exchange, the plan covers most or all of the 20% of costs that original Medicare leaves on your plate.
You see any doctor who takes Medicare, anywhere in the country.
The real math shows up when you get sick.
A 2024 study in JAMA Health Forum found that patients with traditional Medicare plus Medigap had lower out-of-pocket costs on average than those in Advantage plans, especially those needing cancer treatment, skilled nursing, or repeated specialist visits.
Advantage plans cap your yearly out-of-pocket spending — but that cap can run $8,850 in-network for 2025, and higher if you go out of network.
Medigap Plan G, the most popular option for new enrollees, leaves you with essentially just the Part B deductible ($257 in 2025) after premiums.
One hospital stay can wipe out a year of Advantage "savings." Here's the trap nobody warns you about: you can switch from Advantage to Medigap easily in most states only during your first 12 months on Medicare.
After that, insurers in 46 states can medically underwrite you — meaning they can reject you or charge more because of your health history.
A diabetes diagnosis or a past cancer treatment can lock you out for good.
Meanwhile, going the other direction — Medigap to Advantage — is almost always allowed during the annual enrollment window.
Advantage plans are easy to enter and hard to leave; Medigap is harder to enter and easy to keep.
If you're healthy and have modest savings, Advantage can work fine for years.
If you have chronic conditions, travel, or want to avoid fighting for approvals, Medigap often pays for itself.
The worst outcome is picking Advantage for the $0 premium, getting diagnosed with something serious, and discovering you can't buy your way out.
The closing opinion: Insurers design these plans around what looks cheapest on day one, not what protects you on your worst day.
Run your own numbers using your actual medications and doctors before you decide — and if you can afford Medigap in your first year, think hard before giving up that one-time guaranteed entry.
Final Thoughts
It's the rare financial door that only opens once.