The actual data sent is the same, however...
#1 Sends Content-Type:application/json
to the browser
#2 Sends Content-Type:text/html
#1 is more correct but it depends on your Javascript, see: What is the correct JSON content type?
However, it is much simpler to just return the model. It is automagically returned as JSON and the Content-Type is set correctly:
return $model;
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