1. Original Medicare offers access to all of the doctors anywhere in the country who accept Medicare (most do), while most MA plans have only limited regional network of doctors and facilities (much like an HMO).
2. If you enroll in MA then become ill, in the majority of states you likely won't be able to switch back to Original Medicare, that's because it's imperative to add a supplemental private insurance policy to Original Medicare (Medigap) to cover expenses that Medicare does not pick up, and there is only one window - when you first sign up for Medicare - for getting a Medigap policy that covers any preexisting conditions and doesn't charge you extra for the coverage.