When you eventually get to the extension of stay phase using retirement as the reason seems to be easier (less paperwork) but requires more money in the bank. Plus you probably won't be able to work or volunteer, whereas you can if you use marriage.
regarding cancelling the work permit. I recently tried to cancel mine at Bangkok MOL. They said new rule the employer has to cancel, the employee cannot.
Sitting drinking coffee with the wife I asked her how are foreigners supposed to figure out the government. Her reply, what makes you think we Thai have figured it out.