Side-Nav layout features tabs on the left side of the page, making it easier for patrons to navigate and clearly see the content. The challenge with this layout is the entire guide must use the same page layout. The second challenge is the default layout is for one column. To have more options with side-nav layout you need to make custom templates. Research with university students has shown this is the preferred layout.
Tabs Layout has all of the tabs near the top in one horizontal line. This is the layout from the original version of LibGuides. With this layout each page in a guide can have it's own style. It is easy to go from one, two, or three columns and you can make the size of each column whatever you like.