I am an Australian citizen married to a Thai national for 6 years. I persisted with a marriage extension for several years, until I changed to retiirement extension 2 years ago. The main reason for the change was the marriage extension was much more complicated, because my local Nakhon Si Thammarat Immigration had to send all my documents to Hat Yai. The biggest pain in the arse was having to physically take our local village chief to Immigration to certify me! The whole process took about 4 weeks. Alternatively, retirement extension...in and out in an hour, no village chief, fully stamped. So, in my experience, the choice between the 2 types of extension depends on which office is dealing with your application