I would probably write [name_f]Maisy[/name_f].
I have never seen [name_f]Maisy[/name_f], so I would automatically spell it [name_f]Maisie[/name_f] unless told otherwise. I also prefer the look of [name_f]Maisie[/name_f], though I have no idea why! 
Definitely [name_f]Maisie[/name_f]!