Multisite or hierarchical cross-posts-type taxonomies?

I could need a piece of advice on this question, if I should better build my customer’s site using multisite or singlesite.

There are in fact three companies in one group. The group should possess the front page and some news stuff, while the sub-companies get one section (or sub-site) each. They plan to have the same basic design though the whole site. One person should have access to publishing within each company’s section. Each section is planned to contain custom post types, posts and pages.

It seems to me to be quite a job to set up a common company-taxonomy which could separate all content from each company from the other companies’ content, and granting editor access to this content exclusively to one user or user-group? Or is this possible at all?

On the other hand, setting up multisite is also a more demanding task, as is maintaining it. It seems like overkill in this case, if there is a not-too-demanding single-site solution.