different lens construction between the 2 sigma 30mm lenses...the DN has 2 aspherical elements and 9 elements in 8 groups...If I look at the 30mm DC Art lens: The EF-M 32mm is by far the better lens (see the-digital-pictures.com) and I have my doubts that two additional lenses (9 lenses in 7 groups for the EF-M compatible 30mm Sigma) will do the magic trick and lift it to the IQ of the EF-M 32 with 14 lenses in 8 groups. The EF-M 32 is by far the best lens I have ever used just at f/1.4 .
EDIT: And the max. reprod ratio of 1:4 is - for me - a very strong argument for the Canon lens because it is half way to macro while the Sigma will have presumably the same 1:7 like the Sony version.
But you are right that for lots of us some minor penalty in IQ at half the price is fine: As someone who need wide angle very seldom the 16mm will be a good companion e.g. on my old EOS M (in a triple setup with M50 + EF-M 32, M50 + EF 4 70-200 in a small photo backpack - never change lenses anymore).
EDIT: And if it motivates Canon to do a EF-M 1.4 60 mm and an EF-M 2.0 85 IS both with 1:4 max. reprod. ratio - would help to increase choices!
reviews have been quite good so far for all 1.4 DC DN lenses