"One half" vs "a half"

I'm working on a copy editing project and in the copy they use

...only nine and one-half kilometres long...

I have decided the hyphen is wrong. However one half sounds awkward to me. Is that just because of usage? So I'm wondering: are both nine and one half and nine and a half correct? Or is one more formal? In the end, which is preferred?

Regarding the hyphen, you can find a discussion here:

Why does "one half" have no hyphen, but "two-thirds" does?

which clarifies this issue.

Regarding "one half" vs "a half," either are acceptable. It would appear that "one half" is more formal, but "a half" is more popular, not surprisingly as formality has become uncommon these days. :)