I bought an awl from Peter Main, and it's blade is much smaller than the awl blades that you get from tandy or osborne. This awl has to be sharp, really sharp, and it will glide through two layers of 7/8 oz leather easy. The diamond is the best way to go, I think it makes the stitch line look distinctive and perferct. Round holes are fine, but don't make the stitches lay correctly without some manipulation.
I'll try to remember to post some pictures of my awls tonight or tomorrow so you get an idea of the size differences.
Here's a picture of drilled holes with the tinyest drill bit and the holes are still to large, but this was multiple layers (I think 3) of 13/15 oz leather.