The MX was designed by Pentax as a professional camera system, while the ME Super was aimed strictly at the amateur market.
One crucial difference is that the ME Super does not have a depth-of-field preview, while the MX does.
It's hard to beat the MX for all around ruggedness and dependability. Be aware that any camera that you buy that's 20 years old will probably need servicing to lubricate the mechanisms and replace the foam gaskets, which disintegrate with age. You should figure in the cost of a CLA job into the overall price of the camera.
(BTW, I own three MXs. Sadly they haven't been used since I purchased a K10D.
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