I like
Elliott for a boy because it has a softer look and sound - I'd be sad if
Elliott became a girl's-only name and goes the way of
Vivian,
Evelyn, and
Ashley.
I *could* see a girl wearing it, esp. with the nicknames
Elli and
Elle, but the only way I'd condone using it is if it's a family tradition for firstborns to be called
Elliott and you had a girl. I'm all for flouting convention in that case.