I owned the 100-400L, a very good copy of it. I currently own the Sigma 70-200 2.8 OS and the 120-300 2.8 OS. Here are my thoughts.
1. The Sigma 70-200 OS is slightly inferior to the Canon 70-200 2.8 II (and a bit better than the Mk I version). With my Sigma 2x TC it is not as good as my 100-400L was, no surprise there.
2. The Sigma 120-300 2.8 OS is an awesome zoom lens, beats my 70-200 OS in sharpness and contrast. It takes TCs well. The Canon 70-200 II + 1.4x TC would have a very hard time to match the Sigma (without extenders) and is a stop slower too. However great the Canon is, I just can't imagine beating the Sigma with a 1.4x TC attached to the Canon, making it an f/4 lens.
3. The Sigma + 1.4x TC beats the Canon 100-400L in IQ, and matches it in AF speed (based on experience). Plus it is a 420mm f/4 combo, a stop faster than the 100-400L. In this configuration the 70-200 II + 2x TC is clearly beaten by the Sigma + 1.4 TC.
4. The Sigma takes a 2x TC relatively well too, making it a 600mm f/5.6 lens. Now, it will not be as sharp as a Canon 600mm f/4, or 500 f/4 + 1.4x TC, but it is usable, I use this combination all the time. This is a territory the 100-400L or the 70-200 II + TC cannot really touch.
5. WEIGHT - Big issue, you won't walk around for long time with the Sigma 120-300. You can, I did a few times but if convenience is a priority, the Canon 100-400L is much much better. Not having the 100-400L any more, I use my 70-200 OS + a TC for that purpose. not as good but sometimes convenience wins over IQ. the Sigma 70-200 + 2x TC is pretty decent at f/6.3 and good at f/8 (but again not as good as my 100-400L was).
My suggestion is to check out the "Sigma unveils 120-300..." thread. There are many many photos taken with the lens with and without TCs.