I'm sure Canon know the market for 35 f1.4 II; they are absolutely aware of Sigma's offering too so I expect the new lens to be considerably better (built and optics) especially as it will be Expensive!
Canon doesn't to be much much better, if at all (built and optics) than 3rd party, it's sufficient that they're the original with everything that goes along with it - no incompatibilities at all even with newer cameras, in-camera lens profiles, cps support. Given that, many people will happily spend a mere €1000 more.
Also one constant advantage of Canon lenses is the af system which is bound to work better with Canon's pro 5d3/1dx system since it requires firmware lens-camera adjustment (lens groups), for the outdated 5d2/6d af it probably doesn't matter that much.
+1, Canon will have the advantage for pro use, and of course the 'compatibility guarantee' for those of us who don't dare to go third party. Add weather sealing and it's sure to sell well, even if that MkII would not manage to quite touch the Sigma in image quality.
5Dmk3, 5Dmk2, Sony NEX-6 | SY14mm f/2.8, Ʃ20mm f/1.8, 35mm f/2, Ʃ35mm f/1.4A, 50mm f/1.8 I, Ʃ50mm f/1.4 EX, 100mm f/2.8L Macro, 17-40L, 24-105L, 70-200 f/2.8L IS II, 1.4x II, 70-300L, 100-400L | E-mount: 16-50 OSS, Ʃ30mm f/2.8 EX DN, 55-210 OSS | 2x FT-QL, AE-1 Program, some FD(n) & FL primes