I've recently replaced a 160X600 ad with the newer 300X600 ad (text/images), and so far the only image ads displayed are 160X600, centered horizontally. But the CPC is better than it was. I'm wondering if the pool of bidders is larger for 300X600 than for 160X600 -- hence the higher CPC.
Anyone here also use AdWords and/or experiment with this newer format long enough to have a good idea as to whether this is the case? If it is, and if you can afford the additional 140px of horizontal screen space on your site, it would seem to me that 300X600 is the better choice. Also, I'm kind of liking the 160X600 ad centered in the 300px space -- it adds white space to the left and right, which helps the ad to stand out and the page to breathe a bit -- at least on my site.