Of the two, corn cob. The ultrasonic seems to be the best from what I have seen, although I don't have one yet but I plan to in the future.
But if I have a choice of cob and walnut, I would use cob unless you are finding these casings with a leaf blower and they have been on the ground since last year, and in that case use walnut first, then cob.
Add some kind of polish occasionally if you want a shine, but don't add paint thinner unless you are cleaning brass out in the detached out-building or detached garage. That stink will not leave the room if you do it in your living quarters.