If you switch from Medicare Supplemental insurance to an HMO, you cannot switch back, ever.
Caps vary so you need to check your potential plans carefully, not just for any copays/drug costs, but for annual out of pocket and any precursor deductibles.
I've got Medicare Advantage, have had it for nearly 5 years. I've been quite happy knowing exactly what things will cost in advance, having reasonable
premiums that include a gym membership (which is $60/mo otherwise), and some basic dental coverage. The few times I've used it the care has been
quite excellent and appointments have been within one week. If you're not capable of advocating for your
needs, I would go with a supplemental plan rather than a HMO. Squeaky wheels gets the grease...
If you travel, by medical travel insurance.