Rock.
I have several white oaks that I am growing. I have never had success digging during mid-Summer. It may just be me, but Spring works best here in Michigan. The leaf size can vary considerably from tree to tree. Lots of sun will yield smaller leaves. Internode spacing is not tight. Couple that with leaf size, even when reduced, and I find white oak need to be larger in stature to create the proper appearance. The one in my photos was dug in the south and has smaller leaves. It is a big tree and that helps compensate for the leaf size. I happen to like that tree a lot, mainly because it is uncommon to see one as bonsai yet they are very common in the ground here, and also it looks very natural for a bonsai.
Pin oak is another that makes good bonsai. Smaller leaves and good growth habits. Unfortunately, they grow slowly and you can get die back.