just thought i would give you my opinion.
i have a box and i love it to death!!! i also have the small whistle, which i thought i loved in the store, but i guess i changed my mind. the small whistle is a bit smaller and doesn't really have the same shape as the box. it also doesn't hold as much in my opinion. it also doesn't have the shoulder strap, which i find is very useful sometimes.
i am in the process of selling my whistle.
but good luck in which ever bag you choose!