I like both, however, I generally prefer the nns for
Margaret over
Margaret itself. So if a nn would rarely be used, I prefer
Miriam (which is lovely). If she'd be called
Meg most of the time, then I prefer that.
Oh, just realized you have a niece named
Mira. Were it me, I'm not sure I'd go with
Miriam. Then again, if you don't live close to one another, it could work. My sister has an
Emily and her s-i-l asked "permission" to name her dd
Emma. My sister gave it since they live so far apart.