One question to be addressed when designing a site is: “Is it better to have the navigation menu on the side or along the top?”. The answer depends a lot on the content of the site.
Will it have a set number of pages with not much possibility of adding more? If so, then a horizontal menu would work fine since you don’t have to account for the needing the width to add more. If more pages will be continually added, then it’s easier to add to a vertical menu since you can always scroll down the page. The exception to this is if the page(s) will be added as a sub-menu to another page section, then it’s better to display that as a drop-down menu horizontally than one that “flies out” from a side menu.
Another consideration is the length of menu names. Short names are better for horizontal menus due to the fixed width (unless there aren’t many).
Are you selling items that have many different product categories? That would favor a vertical menu to list in a longer line.
Vertical menus, however, take up content space on a page. They are usually found on the left side. Since that is the area most people read first, can that space be better used to promote your business? Space must have be left under the menu for possible expansion. That could leave space to could be used more effectively.