Choosing Between Subpage Default & Subpage Fullwidth
The major difference between the two templates is the size of the page content. The Subpage Default template has a smaller area for page content, with an additional sidebar (including sidebar navigation and room for sidebar sections) on the right of the page. The Subpage Fullwidth template takes up the entire width of the page for content, allowing you to add additional fullwidth sections below.
The advantage to the Subpage Default template is the sidebar navigation. Not only does the redundant navigation help users find related pages easily, but the sidebar navigation is the only place where 3rd level navigation items will be displayed.
The disadvantage to the Subpage Default template is that it is harder to break up the page content to show different segments within the page. This is largely due to the fact that everything will have the same background color.
The advantage to the Subpage Fullwidth template is the ability to use fullwidth sections that help easily break up the page content to highlight different sections of content. Each of these sections allows for an alternate background color (or background image) to help break up the page content, as well as easily control the styling & formatting of the different sections of content.
The disadvantage to the Subpage Fullwidth template is the lack of sidebar navigation. This means that 3rd level navigation items will not show up if they are nested under a page that uses the Subpage Fullwidth template. Additionally, the sidebar navigation will not be there to help users find related pages.