Sylvia, because it's so familiar, and
Silvia, although used by Shakespeare and evocative of silver (and therefore lovely), would be misspelled.
I can just hear the plaintive "
Silvia with an i" all through her life, although I have to admit that
Silvia is a tempting spelling because it seems so much fresher than
Sylvia.
I think I'd stick to the gallic
Sylvie - I love it!