It varies from site to site.
Note: Please feel free to copy and paste this answer into any post where the question involves asking what the best solution is for your own website of which we have no knowledge and would require months of testing to determine what will provide the best results for that website. Anyone who tells you one way is better than another or one type of ad does better than another is talking about their own experience with their own site(s). You might as well ask what the weather is like today. The responses would be just as varied and as unhelpful to you.