@Eric *****
I haven't ever gotten a retirement visa, so I only know what I have been told. I have a friend that gets retirement visas (normally you just renew but he let his visa lapse). He told me that the insurance requirements were different so it was his preference to apply for the retirement visa in country.
I am not sure of other differences.
Differences between METV and retirement? The list is pretty long but basically METV gets you up to 9 months if you work the system, retirement visa is 1 year, no working the system.
"Working the the system" - stamp-in 60 days, extend 30 days, leave/return stamp in 60 days, etc. You only need to leave for 5 minutes (I go to Cambodia and return same day). Effectively you can get up to 9 months. Usually people will choose this option if no other visa option is available.