Do you think Krakauer includes Ruess’ story to emphasize his similarity with McCandless, or his differences?

I think this is meant to highlight the similarities. Krakauer discusses Irish monks who inhabited an island called Pepos near Iceland. These monks created and inhabited stone many hundreds of years before the Anasazi built their desert structures in Davis Gulch. Krakauer connects these monks with adventurers like Everet Reuss and subsequently Chris McCandless for forging their own unique paths in often inhospitable places.