I got this from Pascal (www.29letters.com) by email:
“… anyways, i think it is all visual. Me personally, I do not prefer the straight horizontal one, it is to stiff and might be confused with the dash of the ‘teh’ or… I would go for the slightly slanted one but not the too mush slanted”.
I tend to agree with this though the problam is that the diacritics of many Kufi typefaces look like an afterthought rather than as an integral part of the design. They just don’t fit in…