I have been working with AdSense for several years, so I do earn a decent amount of money with the ads on the site in my signature and more importantly my other websites.. that have basically the same layout. What I noticed is that even when layout was the same, CTR was different for different websites. This is obviously to do with how well the ads are matched with your content by Google. I believe Google constantly works on improving the ads placed on each individual website (and this is why the new AdSense will soon allow us to have more control over the ads on our sites). Which in itself is great news. So the bottom line is, it's probably better to experiment with a few niches at the same time, which is something I'm doing, and then that would allow you to see a greater insight into how/why Google ads work (or don't work). What are everyone's thoughts on this?