Lilia would be the safer option to level since there are few things on our server that can kill her.

Generally speaking, I'd think it'd depend on the enemy you're dueling and what your ranged team looks like. If you have good healers, Lilia will work better. If you've got better archers/witches, Claudia will work, though you'll probably want to raise her skill more. Once princess AW rolls around though, Lilia will leave her in the dust.