Determining whether to use the multisite feature depends on user access and posting. Each site in the network, although sharing the same codebase and users, is still a self-contained unit. Users still have to access the back end of each site to manage options or post to that site. A limited amount of general options is network-wide, and posting is not one of them.
You can use multiple sites in a network to give the appearance that only one site exists. Put the same theme on each site, and the visitor doesn't realize that they are separate. This is a good way to separate sections of a magazine site, using editors for complete sections (sites) but not letting them access other parts of the network or the back end of other sites.
Usually, for multiple users to post to one site, WordPress is sufficient. The multiuser part of the WordPress MU name did not refer to how many users, really. MU was always a bit of a misnomer and an inaccurate depiction of what the software actually did. A network of sites is a much closer description.
Another factor to consider is how comfortable you are with editing files directly on the server. Setting up the network involves access to the server directly, and ongoing maintenance and support for your users can often lead to the network owner doing the necessary maintenance, which is not for the faint of heart. Generally, you should use a network of sites in the following cases: