Iphigeneia in Greek is actually pronounced "if-ee-YE-nee-a". There is no j/g sound.
Both names are beautiful and classic, as well as rare. Personally I prefer the name
Ismene. I think it has a better sound. And while hearing
Iphigeneia, I always think of the girl that was sacrifised by her father for the sake of a war, and this is kind of sad and pessimistic. Yet, it's still a beautiful name.