As a (somewhat overwhelmed and behind-schedule) CMS MS developer, and having just finished a site for a client who wanted it done in Drupal, here are some observations.
Drupal and CMSMS are targeted at slightly different, but overlapping uses:
Drupal is designed from the ground up to be for community-edited sites. It is designed to have users with different roles, and doesn't really differentiate between "front-end" and "back-end" users. Most users are able to submit content, and most users are able to comment on content. Drupal does not impose a hierarchical structure on the site -- in fact, it goes out of its way to avoid that. You can create multiple structures using Taxonomy, menus, etc, but the core concept is more fluid than CMSMS. Drupal has a good module system, a reasonably good forms API, and large numbers of developers. It has a legacy problem of module and template compatibility from one version to another.
CMSMS is more designed for providing content management of a structured site. In CMSMS, there is a separation between "front-end" users who do not maintain or edit content, and "back-end" users who maintain the site. CMSMS has much better support for the admin to maintain templates and stylesheets, and it's much easier to make a custom layout that doesn't look "typical" for the CMS. The module system in CMSMS is, thanks to the new event code, even more flexible than that of Drupal. CMSMS has a smaller development community, but it is very friendly.
I could go into more detail, but this is a high level summary. In both systems, the core is very well written and well thought out. In both systems, the modules are hit-and-miss -- there are some that are buggy, security risks, etc. If I wanted to create a community news portal, I might go with Drupal. If I wanted to enable people within a company to maintain their corporate site, I'd go with CMSMS.
OK, all that aside. If I were going to write a community art site, where there was voting and comments, I'd actually start with Gallery2. I haven't looked at the code, but the feature set is nearly there. It should be easier to hack on than either CMSMS or Drupal.
Well, you wanted opinions. That's mine
