Should I go for Multisite or separate WP Installation?

Generally choosing separate WP installations means that your websites won’t have anything in common.

Choosing Multisite installation enables you some sort of centralization. Your blogs usually are separate (eg. different domains / themes), but they may share something.

According to your description you might want consider a single installation. It is possible to create the described URL structure using WordPress Custom Post Types and Custom Taxonomies. For the registered users functionality, you could adapt the Contributor user role to your needs, so users, who are Contributors are limited only to posting articles to your website.