Of course, it’s a bit hard to know which type you should use so you should probably experiment with both for a while before you decide.
You can also view an example of how the unit will look to the right of your page. However, you may only use three ad units and one link unit on any given page. This is believed to be a step which Google takes for quality control.
The next step is to choose your add format and colors. You can basically select any color palette you choose with Google offering some of its own if you don’t have the time or skill to create one. You can constantly view how the palette will look through the aid of an example. The one that works best in terms of appearance and revenues will vary with the look, feel and content of a website.
However, your ad formats are limited to a choice of eleven formats. There’s an “Ad Formats” link which takes you to a page that lets you see all even of these in action so you can decide best which one suits your site. Sometimes the most intrusive, doesn’t work best however again, this can vary from website to website.