Supposedly both swords are capable to release projected slashes of anti Magic, and the Demon Dweller Sword can also absorb the magic of other people and use those power. What are the advantages of the Demon Slayer Katana then? Is it just like a backup sword with no special attributes compared to the other Asta's swords?
Yes, as far as we know, the Demon-Slashing Katana is just that. A katana with anti-magic applied. This is because Asta's previous swords were created by Licht and embued with special properties, whereas Yami's katana had nothing special and only acts as a channel for magic.