Site Collections vs. Subsites

Create a site collection to support

Create a subsite to support

A security boundary (significantly different permissions and roles) between your sites and other sites

Shared navigation between sites

Delegation of site administration to someone different from the administrator of existing site collections

Inherited permissions from parent sites (parent sites are the higher-level sites that subsites are created within)

Search features or workflows that are scoped differently from existing site collections

Shared lists between sites

A size quota different from that of existing site collections

Shared design elements, such as themes or styles, between sites

Site collection features that are not active on other site collections (for example, the Publishing Infrastructure feature)

A small use of your SharePoint size allotment