While a number of companies offer Medigap insurance, they can only offer policies from 11 standardized plans. Each is simply assigned a letter: A, B, C, D, F, G, K, L, M and N. Some states also offer a high-deductible version of Plan F (although it, too, will come off the list for newly eligible Medicare beneficiaries after 2019).

This standardization means that, say, Plan A at one insurance company is the same as Plan A at another. Be aware, however, not every plan is available in all states. And three states — Massachusetts, Minnesota and Wisconsin — standardize their plans differently.

The most popular Medigap option has been Plan F, which is considered the Cadillac of Medigap plans due to its generous coverage and higher premium.

More from Personal Finance:

The best places to own a home – and pay less in taxes

This debt is most likely to force you into bankruptcy

Investors favor US stocks amid political tensions overseas

And although Plan F (and C) no longer will be available for future 65-year-olds, Plan G provides the same coverage, minus the Part B deductibles. That is, its coverage includes copays, deductibles and coinsurance associated with Part A, along with 80% of your emergency overseas medical care (within limits).

It also includes coverage for excess charges that sometimes happen with doctor's offices — i.e., balance billing, which is when the provider charges you for the difference between Medicare's reimbursement rate and its own charges.

There also will be a high-deductible version of Plan G.

Experts recommend giving thought to how often you use the health-care system when you're considering which Medigap policy to choose.

For example, one option might come with a lower premium because it offers less coverage. Yet if you use the health-care system frequently and that lack of full coverage results in many copays or bills for excess charges, you might end up spending more anyway.