Nahida is more versatile, particularly with her elemental skill. She could apply dendro consistently with it's short cooldown and aimed hold plus she can harvest ingredients with it. No need to directly go near a tree just to harvest the fruits or climb ravines to get the plants. You just take a snap and bam. That and she works with all kinds of elements in her team, with electro, pyro, and hydro adding buffs to her burst at C0. Nilou on the other hand, she's got a decent damage and hydro application but limited team comps. To capitalize with the buff from her blooms, you'd need to stick to hydro-dendro teams only. She's niche but if your not into meta and just want her as a character, then go for it if you have the resources.
