I've been comparing some new formats for pages. Specifically I have been comparing the performance of a page that uses a skyscraper to one that uses a large rectangle.

With my layout, the large rectangle seems to give slightly better CTR and eCPM.

My question is this. Do the exact same ads show in both skyscrapers and rectangles, the only difference being the format in which they are presented. Or do Adwords bidders bid specifically for positions in the different types of ad blocks (such as the advertiser participates in Adwords but only allow their ads to show in, say, large rectangles).