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