Generally speaking it makes little difference which insurance company provides you with a Medicare Supplement. There are 10 Standardized Medigap Plans and every company that provides this type of insurance can only use these plans. So from a coverage standpoint it makes no difference.
Where we might find a difference is how often and how large rate increases might be. It’s hard to have a crystal ball but a few companies are notorious for “bait and switch.” They get you in with a low rate and within a few years stop offering coverage to new applicants, form another company with low rates, and then start everything over again.
A good insurance agent can help you decide what company might be the best now and (based upon track record) in the future.