No ordinary news post ordering
You can do this entirely from the order param using MySQL’s CASE syntax:

{% set newsItems = craft.entries({
    section: 'news',
    order:   '(CASE field_myLightswitchField
                  WHEN 1 THEN elements.dateUpdated
                  ELSE elements.dateCreated
              END) DESC'
}) %}


Note that you’ll need to include that “field_” prefix before your Lightswitch field’s handle since this is a more complex condition and Craft won’t be sure if it should add that automatically.