I like left side because you read left to right. In some countries they read right to left. Make sure ad shows up when page loads so people do not have to scroll down to see it.
I have a 200x200 adsense square, which is blended into the content, that I recently moved from the right to the left. So far I'm getting more clicks than when it was on the right. That being said, my skyscraper - which is on the right - gets more clicks than the 200x200 square. I'm going to test this out for a while, then possible move my adsense around again.