you can decide which one you want to show just by thinking about it ,but there is no option for it. the best you can do is ask google not to show cpm ads on your account.
I've never seen a setting in AdSense to show or not show CPM ads. However, switching the ad type to Image Only is about the closest you can get. I have a couple of sites which get a lot of traffic, but as its mostly from StumbleUpon the click through rates are below .1%, and it does seem that they are showing some CPM ads here. Interestingly enough, I only tend to see the image ads show up on the unit below the content, which seems counterintuitive to me. To my knowledge, Google will show CPM ads almost as a last resort when normal ads are just not working. Since both you and Google have the same goal, namely making as much money off the ads on your site, I'd say just let their algo do its work. If you have enough impressions (70k+ monthly) you should look into a CPM ad provider like TribalFusion.
But as a matter of fact, you can't decide whether CPC or CPM ads to be displayed oin your site. It's all determined solely by Google.
Its true, you can not control the type of ads deliver to your site. however, image ads are normally based on CPM.
You can not Decide which ad to show its all bou your PR and traffic Stats. Google analyse high traffic website and make them their "Authority sites" and show CPM ads... bt there is no available option to choose CPM or CPC.