There should be direct differences between slayers and demons that make one more favorable than the other but will still balance the two out so that one isn't as broken as the other.
Slayers get:
-Breathing styles
-Slayer mark just increases speed and strength a bit
-Are overall faster than demons
-get more money from quests
-get Slayer missions
-nichirn katanas
- breathing speed boost
-Slayer cosmetics male or female
-access to the Slayer hq
Demons get:
-overall strength and base hp up
-bdas
-access to the infinity castle
Which also them to teleport anywhere
-they are more stronger and harder to kill then slayers
-night vision
-demon cosmetics male or female