I've been using CMS's since 2001.  I started with E107, tried Joomla, Drupal, MoinMoin, GetSimple and am currently trying TikiCMS.  My recommendation would be to pick a couple of the more mature CMS's and look at the changes they've made for their major release (1.0 to 2.0).  All of the CMS's I've used have had growing pains and have found out the hard way that the way they were doing things was not the most optimal way.  Instead of comparing one CMS to another, compare the same CMS over major versions and see what changes they've made.

Grouping and prioritizing your questions may help getting the information you are looking for.  Groups, users and permissions are all related in some CMS's.  A user is assigned to a group, groups are given permissions, permissions are additive so each user starts off in a group that has minimal permissions.  What goes into the core is a design issue, so again, I would look at how various CMS's have changed what they put in the core rather then compare one CMS's core offerings to another CMS's core offerings.