I prefer
Isolda. Because
Isolde has that e at the end - I have to guess whether it's pronounced, or not; if it's pronounced, it sounds too exotic for my taste. And because the sound of
Iseult reminds me of "insult."
It fits well with the
Isabella /
Isidora fashionable look & sound, yeah - but I don't think it comes off as too much of an attempt to "uniquely fit in" like
Aubrey,
Avery, Kinsley, or anything like that. Because
Isabella /
Isidora aren't really faddish, so much as they are just old names that are newly fashionable.