netmeg - 3:11 pm on Mar 30, 2011 (gmt 0)
My opinion is that four full days is not near enough time to do a reasonable test.
You have to take into account other things too - how many impressions? How many clicks? Are your ad units set to both text and image? Are both of them capable of displaying text and image? (AdSense doesn't carry image ads for all the available ad unit sizes, for example) I don't mean you have to tell US the answers to all these questions, but you need to take them all into account (and more) before you can come to any conclusions.
And yes, over the years, across the board, the first loading ad unit on most if not all of my sites is the highest earning.