answer
from my personal expierence the large square like banners have the highest CPM rates, followed the the large horizontal and vertical banners. The tiny boxes that can be fit into almost any webpage usually provide the lowest CPM, but it is nothing drastically different.
The theory that there different because there more noticeable might be true, but I think the amount of sites paying for networks ads in the Large squares are greater then the tiny banners, meaning a bigger ad rotation and more counted CPM impressions.