Tha advantage of a 12 month extension is it can be renewed year on year for 1900B, you'll need a re-entry permit when you leave (1000B single or 3800B multi-entry) and you'll never need a Visa again as long as you don't miss an extension
A re-entry permit can be ontained from an Immigration Office or during departure at an international airport. It costs 1100B (single) or 3800B (multi-entry). It keeps your current status i.e. entry stamp valid while you are out of the country and you get stamped back in with the same date.