I was comparing Canon 70-300 IS with 70-200 f/4 IS. These are crops I made for my own tests. Nothing scientific here, but good enough for me.
70-200 was compared with Sigma 1.4 Teleconverter and without it. On the left is 70-200 and on the right is 70-300. Everything with 280mm is with Sigma TC14.
Images taken in RAW, converted to jpeg without any post processing. Both lenses with IS, Canon 30D, ISO 160, speeds above 1/2000, wide open.