What is best to maximize usability: a fixed aside nav or a top fixed nav?

Many websites have fixed menus on the top of the screen so the user doesn't feel lost in long pages that require lots of scrolling. But usually this kind of menu is just a rectangle with text. What if you use a fixed aside area on the left as nav? Would that improve user's experience?

I recently found this HTML template and it has this left menu I said I felt like it was a much better experience than using a top-fixed-nav. So I want to know if this is just a personal preference or a real thing.