KOH (Potassium Hydroxide) is dearer than NaOH. {(Caustic soda) Sodium Hydroxide}
NaOH is easier to obtain in small quantities than KOH.
KOH makes a liquid byproduct, NaOH makes a more solid byproduct.
You use slightly more KOH than NaOH for the same quantity of oil.
Washing can be easier using KOH than with NaOH.
The finished product (biodiesel) is the same no matter which chemical you use, once washed and dryed.

Others may be able to add more to the list.