I´m really curious to see how it performs against 35 Art. I have seen a couple of reviews but it seems no one compares these two. Still waiting for the first user reports and then I´ll decide. I would prefer 35 before 50 focal length for my 5D3. But if this new 50 is optically better then I´ll probably change my decision. Only optical performance matters for me, I do not care about weight or size. Sharpness, CA, micro contrast, this is important for me.
I think Neuroanatomist explained this in some recent thread: most 35s use retrofocal design, whereas most 50s in the market (until recently) use a double gauss design. The double gauss design is simpler and more compact, but allows for fewer corrections and as a result gave lenses that were less than stellar in performance, especially wide open. Zeiss, and now Sigma, changed that by offering 50s as retrofocal designs, and as a result they now have 50s that blow the competition out of the water. But that means only the 50mm competition, other 35mm lenses always used retrofocal design and always had the opportunity to be decent performers.
So what do we have now: we have an outdated 35L that gets outclassed by a very modern 35A, but only by so much, the 35L wasn't all that bad after all. And we have a new 50A that makes its competitors look really old. That doesn't mean the 50A is going to be that much sharper/better than the 35A or the 35L. What the 50A does is give you the option to pick between a decent 50 and a decent 35. The final decision should (and can now) be made based on what focal length you want, not by some 2% difference in MTF ratings.
I agree, I do not expect big differences between 50A and 35A. Differences in optical performance will be most probably negligible. But you know, it would be nice to see a comparison from somebody, who had a chance to play with both of them. Fortunately I´m not forced to make the decision right now. I´ll most probably compare both personally before I make my final decision.