I've been deep diving into this, since I'll start Medicare in a month (and 8 days, but who's counting).
I asked my doctor about Medicare A, B, D vs an Advantage plan. He said "Well, Medicare isn't trying to make money off you". Got it.
Good, free, non-biased info is available -
https://www.shiphelp.org. So much is state dependent, so they're very helpful.
You have 3 months before your 65th birthday, the month of your birthday, and 3 months after to decide what you want. If you don't sign up within that timeframe, there is a
lifetime penalty. Both my brother in laws are paying it. Stupid mistake.
The Medigap plans are standardized, so it's generally recommended that you pick the cheapest (although some do offer some extras, like gym memberships, and a broker can look into their history of rate increases). Once Medicare pays their part, they pay their part - no funny stuff like Advantage plans.
The price of Medigap plans can and will increase, usually with age, also with inflation.
Most friends I've talked to seem to go for Plan G (which pays the 20% after the Medicare deductible) or Plan N (which pays the 20% less some small copays). The company I'm looking at, G is $116/mo at 65, up to $290/mo at 80+. N slightly less.
I'm actually looking at G-HD, which very few even consider. I'll be responsible for the 20% up to $2,800, but it's $36/mo at 65, and $42/mo at 80+. I figure the crossover point is around $5k of billed services, so if I stay out of the hospital, most likely it'll end up cheaper overall. If I do end up in the hospital or get diagnosed with cancer, I'm covered for potentially huge bills.
The company I'm looking at allows you to change supplement plans yearly without underwriting (my state does not), so I can switch to a different plan if this ends up being a stupid idea.
Part D is another decision, although you can change it yearly. If you don't take any expensive meds, you should just get the cheapest plan. Usually there's a free one with a $590 deductible. If you do take something expensive, there are "enhanced" plans with no deductible and some weird rules on what counts toward your $2k yearly limit on prescription cost per year (not including your premium). I'll be paying $95/month, but my $25k in prescriptions per year will cost me <$500. I hope that this new "no donut hole" Part D survives this administration.