I disagree. Marina is the set up princess of peace. It's almost expected she'll die or something to set Setsuna off to fight the good fight. Having something happen to her (especially given that she's involved in conflict) wouldn't be as emphatic.
Louise was just involved in silly, meaningless side stories up until this point. She was living her life. And she was the happy, cheerful, energetic 'genki' girl.
It hits hugely harder that this was Louise, IMO.