I like
Hallie (HAL-ee). I like HAY-lee better, but I still like HAL-ee a lot.
People might mispronounce it, but I don't think it would be a huge issue.
I prefer the -ey spelling because I think it looks less nickname-like that way. Halley.
I know one Halley (age 22).
It's probably less popular because
Hayley is a slightly less harsh sound, so it's the version that people took too more readily.