The difference between AE and MM lenses is just important, if you want to use Tv or P-mode. AE lenses are with 2-3 exceptions just the older technique.
All Contax Cameras before the 159MM, which means befor ca. 1984 offered just Manual Mode and Av. Therefore the lens barrel has had different requirements than for the Contax Cameras after 1984. All lenses produced after 1984 (with 2-3 exceptions) are MM lenses.
The 159MM was the first Contax Camera with the so called Multi-Mode (MM), which means you can use them also for the new introduced modes Tv and P. There is a little switch on The MM lenses, which makes the Tv and P mode possible.
Optically, there is no differences in lens design, it is just the mechanical connection part. You can recognize the newer MM-lenses on the smallest aperture, which is always a green number (i.e. 16 or 22).
If you do not use anyway Tv and P mode, it does not make any differences to you. Used prices for AE are cheaper.
There is another nice advantage of this mechanical change: you can use a kind of selective Tv-Mode with MM-lenses.
If you put in Tv mode the aperture i.e. on 8 and the speed on 1/125, the camera will react as follows:
If there is enough light, you will shoot with speeds of 1/125 and maximum aperture 8. If there is even more light available, the aperture will stay at 8, but the speed will go faster than 1/125. Why is this an advantage?
Because you can use the lens at the optimum optical performance i.e 5.6 or 8 for maximum resolution of the lens and the film. With the speed setting of 125 you know that this will be shake-free. A kind of "do not worry combination".
Unfortunately, this is just because of this technical modification of the AE lens to the MM lenses. Therefore the N-System does not have that advantage anymore, which is a pitty...
hope that helps
dirk