If I install the same module in sites/all/modules vs sites/example.com/modules, which is used?
The sites/site.com/modules will be used over anything in sites/all/modules.

There's a really good description of how it works on Acquia:


  Site-specific modules and themes
  
  Each site in a multi-site install contains at least a settings.php and
  a /files directory. But each multi-site directory can contain modules
  and themes too - and any modules or themes in those directories will
  only be available to that site.
  
  For example, imagine I have two sites, example.com and example2.com:
  
  /sites/all/modules/contrib contains all of the community contributed modules that I want both sites to share. For example, Views might live there.
  
  /sites/all/modules/custom contains all of my custom modules, written specifically for my Drupal installation.
  
  /sites/example2.com/themes contains any themes you want to only be available on example2.com.
  
  /sites/example2.com/modules contains any modules you want to only be available on example2.com.
  
  When Drupal looks for available themes and modules, it searches in the
  following order:
  
  /modules/
  
  /profiles/ 
  
  /sites/all/ 
  
  /sites/[sitename]/
  
  So, when I write my custom theme for example2.com, I can put it into
  the example2.com multi-site directory:
  
  /sites/example2.com/themes/chicken
  
  And the "Chicken" theme will only be available to users of
  example2.com. Users of example2.com will still have access to all of
  the themes in /sites/all and /themes, but example.com won't be able to
  see the "Chicken" theme at all.