Kill me if I'm wrong, but Miriam Margaret as a combo kind of has a special zing to it, in my ears, at least.
I love Margaret. It is my grandmother's middle name, but a lot of people do not like the harsh 'g'.
I think I'd go with Miriam, I love the name. It has such a great flow, and I love Mir or Miri as a nick name. However Maggie and Meg are also fun for Margaret.