Chrispcritters - 10:26 pm on Sep 18, 2013 (gmt 0)
Be careful about focusing on CPC or CTR by themselves. What is important is the overall RPM of the ad location (and the page as a whole).
You generally want your best performing ad location to be the first location that is filled with an ad -- this does not mean that it needs to be highest up on the page. You can do clever things with CSS.