You don't want your ad buried among 20 other ads in an ezine with NO real content. Go for the top spot or somewhere near the top.
People start at the top of an ezine and generally work their way down.
When you are contemplating placing an ad, if the ezine publisher doesn't offer a top spot, email them and ask to have your ad placed near the top. Often ezine publishers are anxious enough to get your business that they will agree to do this just to make the sale. Most people hate saying no to potential customers. So capitalize on this fact by just asking to have your ad in or near the top spot. Headlines of course are very important.