I think
Sylvia is an overlooked treasure; it is a very elegant, feminine and distinguished name, and has no reasons to be so underused. But to me that is a plus, anyway.
I find the Y spelling more appealing, but both are lovely. I don't know if "
Silvia" would be often misspelled in the USA, but I can tell that this is the most common one in Portuguese-speaking countries, although "
Sylvia" is also used.