When you turn 65 it's the only time you can get a supplement at the cheapest price. Then, you get to keep it. If you sign up for a supplement later on, the insurance company will decide what to charge. It's about 200 a month. The good part is that Medicare is your primary insurance and you don't have to worry much about getting authorizations.
An Advantage plan is cheaper if you don't have lots of medical bills. However, the insurance company has to give authorization for almost everything--especially if you have an HMO. With an HMO, your primary has to give you a referral every time you need a specialist.