In my opinion, the "ku-Arab-xxx" whould be better.
Because it will set the Kurdish as a right-to-left display (using CSS).

I agree, because Kurdish can be written in latin script as used in Turkey and Syria,

or Perso-Arabic in northern Irak and Iran

Then we can have ku-Latn for Kurmanji or Yekgirtú (ltr), and ku-Arab for Soraní (rtl)

and Sorani will be recognized correctly as RTL language and displayed correctly

I opened Bug 12250 [1] to add the language and change the name

Regards,

Bernardo

[1] http://bugs.koha-community.org/bugzilla3/show_bug.cgi?id=12250