Dear Henry,
Wiz and I are trying to compare like for like here. The X15 though a very admirable project was essentially the precursor to the STS Space Shuttle which achieves orbital velocity and LEO, something the X15 never did, (though with a little more money - who knows). Scramjet has been a much harder technical achievement mainly due to such problems as materials technology and combustion stability. Not having to lug your own supply of very heavy oxidizer along with you will be the real breakthrough.
TOD