My friend has an ARMS mount on his M14, I have the Sadlak on mine. I checked them both out closely. Fit and finish is quite nice on both. They both have a 3 point mounting system. The ARMS does sit about a 1/4" lower than the Sadlak though, but I don't ever remember it jamming when shooting. I guess it comes down to how your M14 ejects brass.
The front mounting point of the ARMS mount will require you to cut a little piece out of the handguard for clearance for the allen bolt unless you change it to a set screw. I also believe the Sadlak has a bit more length, maybe 2 or 3 extra rails. Sadlak costs a bit more too last time I checked.
I can still use iron sights with my Sadlak with or without scope attached (to a point). I think what ADK means is that with the ARMS mount, a scope/rings will block out the irons when attached. But just remove the scope and rings a your good to go with irons.
Can't really go wrong with either in my opinion.