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ATX is slight lighter, older and smaller.
The moving parts of both model is more or less the same. Unless you really need the reversed feature otherwise I would go for the ATX50. The self adjusted screw on the 50 is nice.
What is the 1st stage in each model because that could explain the huge price difference? Usually 40 is pair with either DS4 or US4 while the 50 is with DST(most expensive of the three).