This is just fresh out of Nuffnang’s blog. You can read the blog post from Nuffnang on their CTR click rating system.
Basically, you can view which is the best performing ad unit from the number of stars. Five is the best and 1 is the least.
As can be seen, on this blog, both unit works equally well for me. However, there are certain blogs that seem to fare better in large rectangle and not so much on skyscrapper.
So, how does this info helps us? Well, I will certainly go for the most stars ad unit and reduce the number of ads on my blog to only two at the maximum. As you can see, most of my blogs only have one ad unit.
The less is better. Less is more.