As touched on already, they're not really comparable products as they're not in the same sector and are for different jobs. A clay bar only 'skims' off the above surface contaminants like tree sap, tar, overspray, industrial fallout, etc. basically anything that's settled and bonded to the top of the paint layer.

Using an abrasive 'polish' to remove these will have varying degrees of success, and some may well remove these contaminants from the surface, but in doing so brings up 2 problems; Firstly, as said, it's also going to be removing a degree of the paint in the process, and it's always best to preserve the thickness of your paint whenever you can for obvious reasons. The second problem is that as those hard bits of crap that were stuck to the surface get picked up on the cloth you're polishing with, they will inevitably start to scratch and swirl the surface. A clay bar, because it's only used with light pressure and with plenty of liquid lubricant, won't allow these hard specs of stuff to mark the paint.

This is why claying is always done before any polishing stage, and especially if it's going to be machine polishing. Can you imagine the machine's pad picking up rock hard specs of old tree sap and then once they're lodged in the surface of the pad proceeding to spin them around the paintwork!!!