Conceptual difference between STRESS and SSTRESS

#1
I am studying the difference between STRESS and SSTRESS optimization functions in the context of multidimensional scaling.

I have encountered this definition of optimization criteria for the SSTRESS:

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My question is : why using squared estimators and squared distances (in MDS or in general), what is the difference between using a least squares method like this:

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from using this:


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