Personally I find XML (with an XML parser to properly do the indentation) much easier to read than JSON and it is a shame that JSON seems to be the default format for DTO activities such as API results.  Glad than MS finally saw the light and included JSON in 2016/VNext

A couple of things to be aware of with XML, All of the nested SELECT statements effectively run independently so are a hidden RBAR so if you have a structure like

<Customers>
  <Customer>
   ...
    <Invoices>
      <Invoice>
       ...
       <Lines>
          <Line>
           ... 
           <SerialNumbers>
              <SerialNumber>
               ...


Then that is a lot of processing overhead.

Secondly, If you have a really large dataset which you have output as a single XML results field, you might have issues with saving the results.  I once created a file for 25,000 rent letters with up to 5 levels of nested data and the resulting XML file was 1.4M elements and wouldn't fit into memory.  I ended up having to save the letter fragments to a temp table and then BCP the table contents to a file.