I've experimented with both at various times...I have a 58mm Canon 500D filter that I used to use when I had a 70-300 IS, and I have a set of Kenko extension tubes. I prefer the extension tubes because they don't affect image quality and offer a bit more flexibility...if you have three different tubes, you have seven possible amounts to increase your magnification. With my Sigma 100-300, I usually use the 25mm tube, but I don't usually try to do magnification close to 1:1 so I may not be the best person to ask


The problem with extension tubes is that they decrease the amount of light coming into the lens. They can also cause vignetting, more so if you're shooting full-frame.