Marc Alhadeff from SonyAlphaBlog tested the new Sigma lens:
The Sigma 28-70 F2.8 DG DN Contemporary (849 euros) is an excellent pro zoom for Sony full frame cameras. In most area it is equal to the Sigma 24-70mm F2.8 DG DN Art (1200 euros) for a much more compact size and reduced price. It even does better in term of sharpness at the short end and in term of flare resistance.
The Sigma 24-70mm F2.8 DG DN Art keep the edge for sharpness at 50-70mm and for the extra 4mm at the wide end that will be required for landscape shooters.
Compared to the Tamron 28-75mm F2.8 Di III RXD (750 euros) , the Sigma 28-70 F2.8 DG DN is sharper, has better background blur, better portrait rendering, better build quality and ergonomics in a little bit smaller size , lighter weight for 150 euros more only