Major concerns:
Sylvie might be too dated for French-speaking countries, and too 'nickname-ish' for English-speaking countries
Elodie might sounds like an unfinished version of
Melody, and might be too rare that ppl will mispronounce it often (the accent and the "e" sound)
Sylvie looks nicer and
Elodie sounds nicer to me;
Sylvie's spelling remind me of silver with a glimpse of elegance, while
Elodie's sound is melodious but not too sweet like '-ia' names