One of the things I note in favour of Storms over Hearts, character-arc-wise, is that Storms starts Emma off in a relatively bad place from which we can play her slowly, haltingly and arduously moving upward, with false starts and slipping down every now and then as appropriate. It's a storyline where a lot of the conflict comes from within, and the setting will, hopefully, reflect and support that.
Hearts, by contrast, starts us off in a rather better place, which, uh... from a narrative point of view, not only means that the conflict will be largely external and the setting will reflect and support that by grinding our faces into it, but it also means that she's starting out in a much better place, and thus has a lot more leeway for her character arc to take her downwards. If you start at the bottom, you can either claw your way up or spiral into a mindless monster (we can hopefully avoid the latter through roleplay). If you start towards the middle of the top, there's a lot less "up" for you to move towards, and a heck of a lot more "down".
... also, bluntly, I like the narrative of someone starting at the bottom of the moral ladder from the bottom and climbing to the top by dint of sheer determination and a long and involved series of learning from mistakes a lot more than that of someone starting near the top of the moral ladder and either blithely staying there or, worse, moving down it as they're forced to compromise with an ugly and cynical universe.
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