Why choose hard grads instead of soft ones?

Because the soft ones hardly have any density to them other than right at the top, by the time you're a quarter off the top of the filter you've almost lost all the nd effect.

People say you should only go for the hard grads when there's a distinct horizon otherwise you'll get a band of darkness across the image where the cutoff is - but at the TP convention last year the MD of Lee filters got us all to hold up a hard grad to our eyes to mimic what the lens would see, and you couldn't make out any visible banding. In othet words, the MD of Lee filters said "Fuck the soft grads, they're sh*t". That's good enough for me