In my experience with skidding metal on rocks- the smoother the better. I wouldn't want the diamonds down because it would definitely prevent good sliding. You are almost never sliding over a broad, smooth rock. Its usually a smallish, pointyish surface. The broad ones aren't likely to reach up and hurt anything under there (making a skid necessary) anyway.

And its not likely you will "wear" it down. People have been dragging relatively thin (3/16") Toyota axles across rocks for years and years and I've never heard of any problems with them wearing down.