By "subscription CMS" I'm assuming you don't want to install the CMS yourself, but to use it as a service, right?
If that is the case, and if you want to create access levels to different kinds of content, then Drupal 7 is a good CMS. If you don't want to install Drupal yourself than http://www.drupalgardens.com/ is an excellent host. You can use their free plan to test the solution (the CMS), but you probably want to have support with one of the paid solutions.
Wordpress is a great CMS as well, but I don't know how to limit reading access to certain users to certain content (without installing a plugin). Note: You can password protect a post, but it's not exactly what you asked... I would suggest you ask Kevin for a link with an how-to before you spend hours learning how to use it in vain.
In my opinion, Wordpress is easier (to administrate) than Drupal as well.