I like
Hallie, it's got a friendly sound. I think I prefer it to
Hayley. Hayleys are pretty common in the
UK but I've never met a
Hallie of any spelling.
The astronomer Halley and his comet are usually pronounced like
Hayley here (although apparently he pronounced it
HALL ee) so that spelling would be confusing. I'd go for
Hallie - maybe as an nn for
Halina or Halide.