Hello ,In MSA both are right,but we use "set" when we are talking about feminine object.
Ex: ست مدارس ،ست عربات
sit 3rabat,sit madares
Six vechiles,six schools.
The other one "sitta" is used for masculine,ستة رجال => sittat rejal (six men)
ستة أقلام => sittat aklam (six pens)
Note:i wrote "sittat not sita because of "الحركات" "