I've had both the M14.ca CASM and the ARMS18.
The ARMS is lower and allows you to keep the rear sight in place. You do have to remove the scope and rings to use the rear sight, but not an issue with QD scope rings, which ARMS does make.
The ARMS does sit low enough that casings will bounce off of it during ejection and cause a jam. The solution, as mentioned, is to cut a couple coils off the ejector and extractor springs (exactly how many is found in the DIY thread) to lessen the force of ejection, which I had to do on both of my M305 rifles.
The CASM was okay, but you do have to lose the rear sight and it does cause the bore of the scope to ride much much higher which you will have to compensate for with a larger cheek rest.
I had originally went with the aluminum CASM but didn't the like way it flexed when tensioned too much. I suspect their all steel version would not flex as easily. My rear sight pocket was a little off axis as well so I got rid of the CASM and went with the ARMS 18 with the ARMS 22 QD low rings