I run PHP 7 by default on my laptop. For new projects, I typically use Symfony or Drupal 8, and launch the built-in servers with bin/console server:run or drupal server respectively.
For legacy sites that require PHP 5, I have PHP installed in /usr/local/php56/bin and use nginx w/php-fpm to serve them. For these sites, I need to run drush under PHP 5, but PHP 7 is setup as default via the command line.
My solution for this is pretty simple, create an alias named drush5.