I prefer
Hayley, pronounced HAY-lee, to
Hallie, pronounced HAL-ee. It just looks and sounds prettier to me.
If I did want a HAL-ee, I'd spell it
Hallie. I can't think of a better spelling.
I'm not sure if people would pronounce
Hallie as HAY-lee. When you look at
Hallie, the intuitive pronunciation is HAL-ee, but then
Hayley is more common, so some people might just say HAY-lee. I'm not sure.
I've never known or heard of a
Hallie. I know of one
Hayley. My husband's secretary, way back in the late 80s, had a baby girl and named her
Hayley.