how to propose when editing content 2 possible media style for an image field?

You could add a separate select dropdown textfield with two options, which are the image styles you'd like, and if they select the alternate one, update the field in hook_entity_view.

 function mymodule_entity_view(array &$build, EntityInterface $entity, EntityViewDisplayInterface $display, $view_mode) {
   if ($entity->getEntityType()->id() == 'node' && $entity->bundle() == 'article' && !$entity->field_image_style->isEmpty()) {
     $style = $entity->field_image_style->getValue()[0]['value'];
     $build['field_image'][0]['#image_style'] = $style;
   }
 }