Down Syndrome:
Neanderthal:
Look at the bulbous forehead of the Down syndrome patient, and compare it to the nonexistent forehead of the Neanderthal.
Look at the heavy orbits and pronounced brow ridge of the Neanderthal, and compare them to the thin orbits and nonexistent brow ridge of the Down syndrome patient.
There are other features you can't see well in these pictures, like the Neanderthal's occipital bun, and the difference in jaw shape. Suffice it to say that ways in which a Down syndrome skull differ from a normal modern human skull are completely different from the ways in which a Neanderthal skull differs from a normal modern human skull.