Margaret Mae is gorgeous, but I agree with PP that people will think of the song Maggie May. As a Maggie, the song already comes up enough without having worry about a matching MN, too!
As for the nicknames, I wouldn't worry about it. I have a strong suspicion that one will stick and the other will probably fade out. That's cool though - whichever one fits your DD best will be her nn!
CONGRATS!