Jason_M & superstring nailed it! They're two totally different products for two different purposes. As Jason said, clay removes bonded contaminents from the 'surface' of the paint. Clay isn't going to remove 'below' surface defects like scratches, swirls, etc. ScratchX contains microscopic diminishing abrasives that allows you to remove 'below' surface defects by gently abrading the surface. As you work the ScratchX against the finish, the diminishing abrasives that do the abrading breakdown and then polish the finish. ScratchX works well, but it takes a lot of effort to make it work well. It's good for small spots, but you would not want to do a whole car by hand with ScratchX as it would take forever and a day to do so.