Mine click now and then as well. I've tightened the steels also. I'd be interested if anyone has any insight as well.
As for the difference, I don't recall there being much. M8s have a hard plastic metatarsal guard on the tongue. Oh and I think a full carbon chassis, versus carbon and fibreglass. For me, there wasn't enough of a difference to justify spending more. But I am a beginner, so feel free to take that with a grain of salt.