purplecape - 3:40 pm on Feb 11, 2010 (gmt 0)
I think my experience runs pretty much counter to your theory, which suggests that, as someone said, it's impossible to generalize.
I had had Amazon for years on my site before I added AdSense. I worked hard to increase my earnings, but when I simply threw an AdSense block on about 1/3 of the pages of my site, I was instantly earning more than I was from Amazon. AdSense grew even beyond that for the next couple of years, and has since dropped off, for various reasons (including the economy). Amazon has grown slowly, interestingly enough, in proportion to the work I put into it, and is now catching up to AdSense. But what seems to work with Amazon just doesn't work with AdSense--for me. I can work on Amazon and gradually build. AdSense success seems to be far less subject to my control.