Every fall, millions of Americans on Medicare face the same confusing fork in the road.
One path promises $0 premiums and extra perks like dental and gym memberships.
The other comes with a monthly bill but covers almost everything.
Picking wrong can quietly drain your wallet for years.
The two options work in completely opposite ways.
Medicare Advantage, also called Part C, replaces Original Medicare with a private plan, often an HMO or PPO.
Medigap, or a Medicare Supplement plan, sits behind Original Medicare and picks up most of what it leaves behind.
That difference shows up fast when you actually get sick.
A Medicare Advantage plan caps your yearly out-of-pocket spending, but in 2025 those limits can run as high as $9,350 in-network before the plan covers everything.
A Medigap Plan G, by contrast, often leaves you with just the Part B deductible, about $257 in 2025, and then pays nearly all the rest.
Medigap can cost $100 to $250 or more per month depending on your age, gender, and state.
Medicare Advantage premiums average close to $0, which is exactly why roughly half of eligible seniors now choose it.
But cheap upfront doesn't mean cheap overall.
Advantage plans use networks, prior authorizations, and referrals.
A 2023 KFF analysis found that about 6 percent of prior authorization requests were denied, and patients who fight those denials win only about 80 percent of the time.
Meanwhile, Medigap lets you see any provider nationwide who accepts Medicare, no referral required.
There's one catch that trips up a lot of people.
You get a guaranteed right to buy Medigap only during your six-month Medigap Open Enrollment window, which starts the month you're 65 and enrolled in Part B.
Miss it, and insurers in most states can charge you more or reject you outright based on health history.
Someone picks a $0 Advantage plan at 65 because it looks like a bargain.
Five years later, after a cancer diagnosis or a heart condition, they want to switch to Medigap.
In 46 states, they may be denied or quoted a price they can't afford.
A few states, including New York, Connecticut, Massachusetts, and Maine, offer year-round guaranteed issue, so residents have more flexibility.
Everyone else should treat that first enrollment window as a one-time, irreversible decision.
It depends on your health, your budget, and your tolerance for surprises.
If you're healthy, want low premiums, and don't mind networks, Advantage can work well.
If you want predictable costs, broad access, and you can swing the monthly premium, Medigap tends to protect you better as you age.
Run the math on worst-case years, not just the average ones.
Check whether your doctors are in-network.
Read the formulary for your prescriptions.
And if you're leaning toward switching, do it before your health gives an insurer a reason to say no.
The bottom line: this isn't really a choice between cheap and expensive.
It's a choice between paying now and paying later, and later usually costs more.
Final Thoughts
Whichever way you go, make the decision with your eyes open, because you may not get a second chance to change your mind.