To understand the choice, start with what Original Medicare leaves uncovered. Medicare Part A (hospital) and Part B (medical) cover a great deal, but they come with no annual out-of-pocket limit. In 2026, Part B carries a standard premium of $202.90 a month and a $283 annual deductible, while a hospital stay under Part A starts with a $1,736 deductible per benefit period. Crucially, after you have met those, you still owe 20 percent of most Part B services with no ceiling — a serious illness could mean unlimited bills. Both Medigap and Medicare Advantage exist to solve that exposure, but they solve it in opposite ways.

A Medigap policy, also called Medicare Supplement insurance, sits on top of Original Medicare and pays the gaps — the deductibles, the coinsurance, the 20 percent that would otherwise have no cap. You keep Original Medicare, which means you can see any doctor or hospital in the country that accepts Medicare, with no networks and no referrals. The policies are standardized by letter (Plan G and Plan N are the most popular for new enrollees), so a Plan G from one insurer covers exactly the same things as a Plan G from another — you are really just shopping on price and service. The trade-off is cost: you pay the Part B premium plus a Medigap premium that often runs $130 to $300-plus a month depending on your age, location, and the plan, and you still buy a standalone Part D drug plan separately.

Medicare Advantage (Part C) takes the other road entirely. Instead of supplementing Original Medicare, an Advantage plan replaces it: a private insurer is paid by the government to deliver your Part A and Part B benefits, almost always bundled with Part D drug coverage and frequently with extras like dental, vision, hearing, and gym memberships that Original Medicare does not provide. Many Advantage plans carry a zero-dollar premium beyond what you already pay for Part B, which is the headline that draws people in. The cost shows up elsewhere: you pay copays as you use care, you are generally limited to the plan's provider network (an HMO or PPO), specialist visits may need referrals or prior authorization, and the plan can change its network, drug list, and costs every year. The protection is an annual out-of-pocket maximum, which caps your exposure — but that cap can still run into the thousands.

The single most important thing to understand is the asymmetry of switching, because it quietly drives the whole decision. When you first enroll in Medicare at 65, you get a one-time Medigap open-enrollment window — six months beginning when you are 65 and enrolled in Part B — during which an insurer must sell you any Medigap policy regardless of your health history, at the best available price. This is the only guaranteed shot most people get. If you instead choose Medicare Advantage at 65 and later decide you want the freedom and predictability of Medigap, that protection is gone: outside of a few states with special rules, the Medigap insurer can require medical underwriting, raise your premium, or deny you coverage entirely based on conditions you have developed since. Going Advantage-to-Medigap is often the hardest move to reverse in all of Medicare.

Cost comparison is where people get misled, because the two designs front-load and back-load differently. Medigap costs more every month but very little when you actually get sick — it is insurance you can budget around to the dollar. Medicare Advantage costs little or nothing each month but more when you use care, and the bills are least predictable in exactly the years your health is worst. A useful way to frame it: Medigap is the choice that trades higher fixed cost for low and predictable variable cost; Advantage is the choice that trades low fixed cost for higher and less predictable variable cost. Neither is cheaper in the abstract — it depends entirely on how much care you end up needing.

Drug coverage and the 2026 rules matter to both. Under the Inflation Reduction Act, every Part D plan — whether standalone (paired with Medigap) or built into an Advantage plan — now carries a hard annual out-of-pocket cap on covered prescriptions, which rises to $2,100 in 2026. That is a meaningful protection for anyone on expensive medications, and it applies regardless of which path you choose. Higher-income retirees should also remember IRMAA: in 2026, a modified adjusted gross income above $109,000 for a single filer or $218,000 for a couple adds a surcharge to both Part B and Part D premiums, based on your income from two years earlier — a cost that exists no matter which coverage route you take.

There is no universally right answer, but there are clear tendencies. Retirees who travel often, split time between states, want their pick of specialists and top hospitals, or simply value knowing their costs in advance tend to be happier with Original Medicare plus Medigap, and can afford the higher premium. Retirees on a tighter monthly budget, who are comfortable within a network, value the bundled extras, and are generally healthy often do well with Medicare Advantage — as long as they go in understanding the network limits and the difficulty of switching back later.

Treat your initial enrollment at 65 as the highest-stakes coverage decision you will make, because the Medigap guarantee you get then may never come back. If predictable costs and provider freedom matter to you and you can afford the premium, the safest long-term move is to lock in Medigap during that one-time window when no insurer can turn you down.

If Medicare Advantage's low premium and bundled benefits appeal to you, go in with clear eyes: confirm your doctors and medications are covered, understand the network and referral rules, and know the annual out-of-pocket maximum — and recognize that returning to Medigap later may require passing a health review you cannot count on.

Whatever you choose, review it every fall during open enrollment (October 15 to December 7). Advantage plans change their networks, drug lists, and costs annually, and standalone Part D plans do too — the plan that was best this year may not be next year.

• Choosing Medicare Advantage at 65 without realizing that switching back to Medigap later can require medical underwriting — and can be denied.
• Missing the one-time six-month Medigap open-enrollment window, after which guaranteed-issue protection generally disappears.
• Comparing only the monthly premium. A zero-premium Advantage plan can cost more than Medigap in a year of heavy care once copays and coinsurance add up.
• Failing to confirm that your specific doctors, hospitals, and prescriptions are in an Advantage plan's network and formulary before enrolling.
• Setting coverage once and never revisiting it during the annual fall open-enrollment period as plans change.