I'd put
Posy /
Posie in the same category as
Daisy,
Poppy and
Clover but not
Marigold and
Magnolia. The latter just 'seem' more sophisticated, more substantial to me. Maybe I'm slightly biased because they literally contain more letters...I don't know exactly.
The actual word
Posy /
Posie is not appealing to me at all, sound-wise (cosy, nosy...). The look is weird too. The more I'm sitting here analysing it, the harder it is getting for me to see it as a name and I don't know what spelling to choose!
I guess it comes down to, do you normally prefer names spelled with a -y or -ie ending? Do you prefer
Milly or
Millie?
Lucy or
Lucie? etc.
If I am forced to pick one, I guess it would be
Posie. It looks more name-y than Posy...I don't know. What about Posey?
I'll end my rant of disgust now :)