I think I currently prefer
Lucy. I used to really like
Maia but that has passed a bit. I still like it but not as much as
Lucy. I used to prefer
Maia but now I much prefer the spelling of
Maya. But both are fine as long as it's not
Mya which I think is awful. They are both nice and you can't go wrong but I would go with
Lucy. The good thing about
Lucy is that there aren't so many popular alternate spellings. There is
Lucie but it isn't really used much. On the other hand
Maya,
Mya and
Maia are all used quite a bit. She would probably often have to correct the spelling because people would assume
Maya. There is also
Laia in case you want a bit of both ;)
Kaia is also nice or
Aya!