Scathach Vs Karna?

Karna for long term investment imo.

depends on if you need an AOE(Karna) or single target boss killer(Scathach). personally I'd level Scath simply 'cause I don't like Karna's Artworks and my Lancer Alter does Karna's job. but honestly it depends all on what servants you have

Scathach is better in a vacuum but is held back by being quick focused in a world quick support just doesn't exist so once Merlin comes out Karna becomes much better than her. Karna gets a solid anti divine modifier on his NP which makes it suitable for bosses as well. Should a decent quick support ever come out Scathach will likely be better but until then Karna is the stronger servant.

Karna is for long term investment. Scath is obviously better than Karna, but that's only up to the end of part one, beyond that, nah. Quick support aside, she struggles to fill her NP, making her unsuited to take on gauged HP bosses, but if you really like her and you have an idea on how to fix her NP problem, feel free to raise her for long term investment.