The significance of beheading in islam is? The body can not enter their islamic paradise without the head & body attached? Geezers another pagan tribal ritual!
Just guessing here, that the beheading is traditional (what other method would they use - garroting?) and the crucifixion is a method of displaying the corpse for a period of time.
Or maybe it's them virgins would be turned off by the no head thing. Who knows...