The main difference between the DW M-12 and M-15 is the barrel shroud.
The M-12 has what is called the "pork chop" shroud which has an extension
that fits on the front of the frame, with a small dimple that matches a corresponding stud on the frame, the M-15's shrouds are straight with no extension. Also the first model DWs had an outside barrel nut, the shroud had no recess for an internal barrel nut. The two styles of shrouds are not interchangeable, the barrels are. The M-12 had larger hex head sidescrews, the ones from the M-15 do not fit, too small. Also the M-12 had a different style rear sight with easily adjusted screws, designed for a coin rathes than
a screwdriver. The barrels are interchangeable between the two, to the best of my knowledge the lockwork is identical. I have had my M-12 for 30 years, my M-15 for 26, don't worry about resale value, you won't want to sell it.