I debated the same decision a year ago also coming from the 100-400. After trying a Sigma 120-300 I went with the Canon 300 2.8 IS. I only took a few shots with the Sigma so I know I didn't give it a fair test but I just felt more comfortable shelling out the cash for the Canon even without trying one first, The Sigma test shots looked ok but the guy at the camera shop said the focus would be faster on the Canon. I also felt like I knew what to expect from Canon quality wise. My copy works very well with a 1.4 II converter for everything including shooting eagles in flight. My copy takes good pictures with the 2x for baseball, batters in the box, and non fast moving stuff but the autofocus really slows down with the 2x converter. I had the 2x mkii and upgraded to the 2x mkiii, both perform the same as far as autofocus is concerned, crappy. I use mine for birding, baseball, football, soccer and wildlife on my 1D MKIV with and without the 1.4. I have no complaints and don't miss not having the zoom.