I would imagine that this has been asked before, but i couldn't find it.

What is the difference of a Multisite Drupal install vs multiple single site installs on System Resources? Is the PHP memory limit per Drupal Install or per a site in a multisite install? We would have large file uploads via the browser (25-100mb). How does the PHP upload limits effect system resources.

I am building out a church site that will have online donations and an online bookstore. The main set up looks straightforward using Ubercart for the Bookstore and the new Ubercart Donations modules by rszrama for online donations. Thsi would be my first Ubercart site.