val vs def

hi there,

when writing code, i often have the choice between this:

 def rotatedLeft(xy: XY) = {
      def update = {
        def xyRotatedLeft = xy -> myWheels(xy).rotatedLeft
        def touchedRotatedRight = xy.touched.flatMap(touched =>
myWheels.get(touched).map(wheel => touched -> wheel.rotatedRight))
        xyRotatedLeft :: touchedRotatedRight
      }
      Wheels(myWheels ++ update)

and this:

 def rotatedLeft(xy: XY) = {
     val update = {
        val xyRotatedLeft = xy -> myWheels(xy).rotatedLeft
        val touchedRotatedRight = xy.touched.flatMap(touched =>
myWheels.get(touched).map(wheel => touched -> wheel.rotatedRight))
        xyRotatedLeft :: touchedRotatedRight
      }
      Wheels(myWheels ++ update)

is there any reason to prefer one over the other if every method is only
called once/every val is only used once?