It's not really that black and white. You'll have to do some testing, because there's no one scenario. It depends on blending. It depends on niche. It depends on placement. It depends on your traffic. In short - it depends.
What netmeg is saying in part is that you can't compare apples with oranges. A banner ad at the top of a page will perform differently than one at the bottom. That goes for different sized ads placed in the same location on a page. You can try one size for awhile then another but because, seasons change and advertising trends change over time it's better to set up an A/B test to see which performs better during the same time period.
Where you put the ad on the page impacts its effectiveness, the color of the ad in relationship to your page, what else is around the ad. All this matters. It's not a simple thing but some, I'm sure, do get lucky on the first try. Some call it a science, I like to think of it as alchemy.