Advertisers place a great deal of focus on the title and description in their ad copy, but the url is often overlooked. The display url of your ad also has an effect on whether or not people are likely to click on your ad. If you use a display url that contributes to a high click through rate, you will in turn be paying a lower average cost per click and saving money. Testing display url's is a great way to increase your click through rate.
Both differing domain names and differing url extentions should be tested. The domain name you use should be very relevant to what you are advertising. If you advertise a wide range of products or services, it may be worthwhile to set up several domains, each with a relevant domain name to a specific product and use these in your advertisements.
You should also be testing different url extentions. For example if Ten Golden Rules has an ad group set up to advertise SEO Consulting, rather than just including the top level domain (www.TenGoldenRules.com) as the display url, variations should be tested, such as (www.TenGoldenRules.com/SEO-consulting).