I will bet most of these people saying the sigma will have problems are canon fanboys. I never owned the 120-300 but always read it was a fantastic lens. I used to own the 100-400v1 and sold it few years back. Got a sigma 150-600C in the spring and it is my favorite lens. Most of the people who put down the sigma lens are fanboys and never even used one before to make any real judgement. My vote would be the 120-300 or the 150-600. If budget tight get the 150-600C. It's IQ is pretty much equal according to sigma. Mine is very sharp wide open. It just isn't built as tough for nasty weather.
I am often partial to Canon lenses based solely on my (somewhat limited) experience with non-Canon lenses.
That being said, if I were looking for a lens to purchase (not rent) longer than 200mm, I'd pick the Tamron 150-600 VC. I rented it a year or so ago for a golf tournament and I was very pleased - right up to and including 600mm. Of course, it was a bright sunny day and I could bump the shutter speed and stop down etc. as needed. But I was delighted with the images and I really loved the lens.