Margaret will in all likelihood be my first daughter's name as well. So good choice :) It's a name with a strong family history for me (my mn, for one). We'll call her Maggie because I insisted on Margaret and told DH he could pick the nickname, and the only one he likes is Maggie. I prefer Margo, but Maggie is nice too.
I think of Margaret as a nerd. She's got a good head on her shoulders and she's sure of herself. As a little girl, she's precocious. Later on, her nerdiness might take the form of a quiet math geek or an outspoken activist, but whatever she does, she is well-informed and smart as a whip.
Maggie, on the other hand, is kind of a blur to me. The nickname has less personality in my mind than Margaret, probably because it's common enough that I've known enough Maggies with enough different personalities to keep me from making snap judgments about it.