Multi or single?

I am fairly new to WP and cannot decide if I need WordPress MS or simple WP for my purposes. We would like to build a congregational website with lots of static pages, and some blog-style info, however, we also need at least 3 sub-sites for some groups who belong to the congregation but have their own issues. I’d like the site to have a consistent look, therefore would use the same theme for all sub-directories but want all sub-dirs to have individual headers, menus etc. We don’t really need the multiple-user feature as the site is mostly informative, so far only one group is interested in having a private forum installed on their sub-directory. In this case, do you think it is better to go with a simple WP installation and create a static page and child-pages for each group (I fear this would limit our options for expansion) with child-themes, or is it better to use MS (and still maintain the one-organisation look and feel)?
I hope I was clear enough. If not, I can explain in more details if anyone is interested.