A few other reasons why I have a Medigap plan instead of Medicare Advantage plan:
1) If you plan to travel outside of your immediate coverage area, most MA plans will provide no coverage while traveling....except for emergency treatment but follow-ups to that emergency treatment are not covered unless you go to your home area to get it. With my Medicare/medigap policy I have nationwide coverage without any restrictions.
2) Deductibles/Co-Pays/Max Out of Pocket limits can be very high with many Medicare Advantage plans but that UHC-AARP medigap plan you mentioned is (based upon the premium amount) likely a Plan G which has very good coverage. I have a Plan G and this year I am responsible for the annual Part B deductible which is $226....I have no out of pocket cost for the rest of the year after paying for $226. With most MA plans, you could be on the hook for $thousands should you need to be hospitalized or have a major health issue, like heart attack, stroke or cancer. My wife has a MA plan and she got cancer. The immunotherapy costs alone were $18K every 2 wks....and that's not counting the chemo and radiation costs. We had to pay $8K last year and again this year before her insurance kicked in anything for it. If she had my plans we would have had to pay $226 in each of the last 2 years and not a penny more.
3) My Part D Rx premium cost is only $11/month. What I found out by using the plan search tool on the Medicare.gov website is that, contrary to popular belief, the lowest total annual cost (premiums + Rx costs) Part D plans often are the lowest premium cost plans....not the higher premium cost plans.
All I can say is, don't buy on premium costs alone or you will likely end up being highly disappointed. Make sure you clearly understand what your total cost exposures will be for the options you are considering and look very, very, very closely at the benefits and benefit limitations. I think you will be surprised by what you find out.
Maybe a MA plan might be great for you. It was for my mother and it (so far) is good for my brother. So I'm not bashing MA plans. Just know what coverage you will get before you sign up for it. And always remember: We do not buy insurance (any kind) because of what our current situation is. We buy it for the "what if" that might happen in the future. And as we age through retirement the risk of encountering a major health issue increases pretty significantly. I prefer to pay a little more (~$200/month for medigap, Part D and dental...vision insurance often is not worth it) for superior coverage now than to pay a lot later for low premium cost coverage now.