There can be many other costs tied to medicare advantage plans, aside from the premiums. Some plans have low premiums but annual deductibles and other out-of-pocket costs like co-pays can be high. You should also check the co-insurance, which is the percentage that you pay for a medical service after the deductible is met.
Check hospitalization coverage and out-of-pocket limits also. For instance, some plans pick up all costs of a hospital stay after 10 days, but most people are released from a hospital before then. Out-of-pocket limits, on the other hand, cap the amount that you have to pay through things like co-pays or co-insurance in a given year.