What are the advantages of a marriage-based visa versus a Non-O A visa in Thailand?

Retirement extension is also easier to get in that you can get the same day you apply where as Marriage extension takes a month to get approved. Also loss paperwork for a Retirement Extension.

Okay this is ONLY the difference from VISA to VISA (O-A to O), and NOT extensions of stays you get inside the country. That's soemthing else entirely.

Aside from the financial requirements (and the police report background check you need for an O-A) the difference between a year long, multi-entry Non-Immigrant Type O-A (Long Stay) visa and a year-long, multi-entry Non-Immigrant Type O visa based on marriage are;

-An O-A and an O visa are both good for a year from the date they're issued

-An O-A visa is good for unlimited entries during that year with each entry getting you stamped in for a whole year

-An O visa is good for unlimited entries during that year with each entry getting you stamped in for 90 days

-The maximum in country time you can wring out of an O-A visa is TWO years (in one year increments) .

-The maximum in country time you can wring out of an O visa is 15 months (in 90 day increments)

Seeing that you hit your 50s in a few years I would say read the website of the Embassy where you like to apply, keep an eye on this page when there are questions about the Non Immigrant O-A (Long Stay) visa or about the Non Immigrant O visa based on marriage with a Thai citizen. Some people claim that Immigration rules changes daily, to tell you the difference between two visa's over the next few years will be polishing the crystal balls to many times.