Canon and Sigma both have a wide range of qualities. From cheap to very good. You have to ask about specific lenses.
I'd agree with this. Further, Sigma makes plenty of lenses for which there is no direct Canon equivalent, like these:
Sigma AF 100-300mm f/4 EX HSM APO
Sigma AF 120-300mm f/2.8 EX HSM APO
Sigma AF 80-400mm f/4.5-5.6 EX APO OS
Sigma AF 135-400mm f/4.5-5.6 APO RF
Sigma AF 15-30mm f/3.5-4.5 EX DG
Sigma AF 17-70mm f/2.8-4.5 DC macro
Sigma AF 18-125mm f/3.5-5.6 DC
Sigma AF 18-200mm f/3.5-6.3 DC
Sigma AF 18-200mm f/3.5-6.3 DC OS
Sigma AF 20mm f/1.8 EX
Sigma AF 30mm f/1.4 EX HSM DC
And of course Canon has plenty of lenses for which Sigma has no equivalent.