Hi,
I pondered both combinations prior to making my purchase.
I found an excellent site (but I can't remember where!) that allows you to compare samples of lenses, and lenses + TC with other lenses, as well as reading a LOT of reviews.
The two were actually quite comparable around the 400mm mark (top end of the 300mm + TC) but the sigma looked a little soft towards the 500mm end.
Does the Canon not give you a stop more? At what point does the sigma stop being 5.6? I am assuming its well before 420mm?
For me the right decision was the 300mm (yet to buy the 1.4x as I can't find one!) because I have the 70-200 range covered already, and the 300mm prime @ F/4 is just stunning, not something I would have got with the sigma.
I guess it comes down to what you use the sigma for... if you find you're shooting in the 400-500 then I would say that is the right choice for you, obviously if most of your shots are 300-400 then the canon could be better.
Do you use it below 300mm ?
They're the questions I would be asking myself, rather than the IQ as there isn't a HUGE difference if your sigma is a nice copy.
If you have a 1.4 or just want to try out the 300mm then let me know as I am only on East Dartmoor.