You've already owned a couple of Sigma's and love them so far- so what makes you think that the 70-200 OS will be any different ?
I don't have a Canon 70-200 IS/non-is/mk1/mk2 to compare it to- and never have had. I do have the 70-200 Sigma OS-and so far, love it like my other Sigma's. Depending upon how you would use it- is it fair to compare an optically stabilized lens to a non-optically stabilized.. Just sayin...
IMO, like I currently do now, I would buy the Sigma and not even look back. The EX series seems to be quite good lenses, especially for the money.. Like most people, having two lenses of the same range does not make financial sense to me. Maybe that will be different after I win the lottery.
Odd thing is: Has anyone seen any of these Sigma's on sale in the sales forums?
EOS 5D III, EOS 7D,EOS Rebel T4i, Canon 70-200 f/2.8 IS II, Canon 24-105L, Canon 18-135 IS STM, 1.4x TC III, 2.0x TC III, Σ 50mm f/1.4, Σ 17-50 OS, Σ 70-200 OS, Σ 50-500 OS, Σ 1.4x TC, Σ 2.0x TC, 580EXII(3), Canon SX-40, Canon S100
Fond memories: Rebel T1i, Canon 18-55 IS, Canon 55-250 IS, 18-135 IS (Given to a good home)...