They are almost identical in how they function except that the MD-11 does not turn off the camera's meter and will drain the motor batteries overnight...the MD-12 will time out and shut the meter off after some time of non-use...
The MD-12 is the better of the two but to be period correct if you're a collector some of the older FM's & FE's will have to have the MD-11...
I have an MD-11 and like it but I have to remember to turn it off when I'm done...
What is the major difference between the MD-11 and the newer MD-12 ? Operation-wise, when you press the shutter release on single mode, the shutter will trickle without advancing to the next frame, only when you release or lift the trigger will advance to the next frame on the MD-11. MD-12's single mode, when press release, the shutter will immediately advance to next frame EVEN if finger is not lifted up from the trigger button and get you ready for the next shot. Next is the power saving feature, the newer MD-12 has a internal circuit to turn off the power 60 sec after inactivity. The MD-11 has no such feature and it requires you to switch off the power to avoid draining the power from the cells.
From the mir.com website...