US 6 should most likely remain primary in Nevada for its lower connectivity and usage by traffic, except for the section between 318 and Ely, and the segments concurrent with US 95 west of Tonopah and 50/93 east of Ely. 

I'd also say the same for the US 93 section upgraded to trunk between Crystal Springs and Majors Place junctions—it's a very comparatively low-traffic route only linking smaller towns, and most traffic travelling between Las Vegas and Ely will use 318 over the less-direct route via Caliente, Panaca, and Pioche.