I do not have much to offer other than my own personal anecdotal experience: Without major health issues (assuming/hoping that will continue for a while), I doubt if you will notice much difference between the two. I have bounced between HMO's and PPO's for years and never really noticed much difference.
If I were in your shoes (so close to Medicare, healthy, etc.) I'd probably choose the HMO based on my past experience. However, the PPO does offer the peace of mind from more [possibly just perceived] flexibility if something should go wrong.
Make sure any doctors, facilities, any expensive medicines, etc. that you use (if any) are covered/preferred on whichever plan you choose.
My current company started offering a high deductible/HSA-eligible plan a few years ago; this is what I am covered by now. (Unfortunately, this seems to have incented me to not see a doctor a couple of times when I probably should have.)
If there's one thing in my life that's missing; It's the time I spend alone
Sailing on the cool and bright clear waters; There's lots of those friendly people
Showin me ways to go; And I never want to lose your inspiration