To be honest, I like them both very much!
That said, I’d go with
Mary. I have it on my PNL, actually.
I like
Margaret but I don’t really like any of its nicknames, and I feel a nickname would be inevitable for
Margaret.
Mary doesn’t have any obvious nicknames, nor does it need one and it’s a lovely name with dozens of great namesakes. It doesn’t sound hopelessly boring or uptight to me like it may to some others. I think it’d be extremely refreshing to meet a little
Mary today.
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Used to be Azula
My ratable list:
https://www.behindthename.com/pnl/174843/136932