I use a Mac for Drupal development and am a long time user of Drush.
Getting ready to develop a new Drupal 8 site and installed via "Option A" as described https://www.drupal.org/docs/develop/using-composer/using-composer-to-manage-drupal-site-dependencies
Following those instructions "out of the box", I wind up with a Drush 9.x instance inside of the Drupal 8 repository.
It is my understanding that Drush 9 won't support Drupal 7. So, I'm forced to have multiple versions of Drush unless I run one global instance of Drush 8.x. At least so it seems to me.
I'm also trying to make the shift to composer and installed Drush 8 as documented here.
And sadly, that instance breaks unless I list the mysql host as "127.0.0.1" (instead of localhost). And if I do that, Drupal breaks.
So - first question: 1) What is are the recommendations for Drush installation on a Mac running multiple versions of Drupal. Is it preferred to run on global instance? (Drush 8.x) - or leverage composer to install Drush locally within each Drupal instance.
2) Why (maybe out of the scope of this discussion) do I suddenly get an error installing Drush 8 using composer on my Drupal 7 instance? Drupal 8 fails as well, but a different error.