I have both. Leaning strongly towards Sigma.
Colours, both good, but imo the Sony lacks a bit saturation and dims some colours to much. Similar like Sigma 14-24mm DG DN the new nano porous coating does very good job without making to vibrant colours.
Contrast: Sigma. Not as good as their primes or 14-24mm, but better then the Sony.
Vignetting: at 24mm wide open it's 'big' the vignetting on Sigma. It's still an issue on the Sony (so neither go free) but Sony better here. Staying F8 at 24mm cures most of issue (and for portrait like stuff i don't mind).
Chromatic aberration: Sigma way better, at first i didn't see any but now i see some traces, but they are incredible small, even better then some badly corrected lenses + software correction. My sony had among worst very broad purple outlining. And it happened in almost every scene, while the Sigma sometimes shows noone at all.
Focus speed, little win to Gmaster (but Sigma is by no means bad, it's better then anything in DSLR landscape imo apart oem pro lenses. No experiments on people tracking yet, sony probably slight win.
Weight: I thought i wouldbe small difference, but the Sony is longer and heavier, and this combines into a less balanced setup, where you actually feel the difference. Now to be fair i like a bit of heft to keep combo steady (and usually the image quality it brings), but the Gmaster imo lacks a bit of that last.
Sharpness:
A7III/A9 no difference (other then listed above)
A7RIII: bad copies of Gmaster (wich i think mine if one of many off) start to get soft a bit at 24mm, but a lot so at 70mm. 70mm is really bad at infinity. The whole gmaster is good at portrait distance though. Sigma way better at infinity (especially 70mm, but surprisingly 24mm is also sharper. The corners get a massive boost above F5.6 on Sigma, this usually is not case on standard zooms, where the corners have unfixable astigmatism (that stopping down barely masks), this is not the case here. Close up i need more time with Sigma, but i think it's just a little less sharp but more then good enough (and has that nice close focus at 24mm). 70mm much better on Sigma.
Sony A7RIV: Same as above but difference really starts to show at infinity. Especially 70mm infinity is night and day better on Sigma. On cloudy days the vignetting on Sigma causes more noise (noise appears faster on r4 small pixels), good sunlight like in summer: no visible difference. I feel comfortable using the Sigma on 60 megapixel, but not so for Gmaster. (Yet the sample Gordon Laing used for his test was much better).
Build quality: no difference overall. Sony zooms stiffer (definitely need two hands), Sigma has just about right resistance. It has all buttons Sony has so no missing (unlike Tamron) and i use all of them, especially AF/MF switch (cause in AF-C you cant magnify and going MF switch is easiest way to engage).
Price: Sigma way better then Sony. And... with seemingly almost no compromise except the heavy vignetting at 24mm. They also use 6 FLD glass element (top level glass, not used in cheap lenses). I see no reason for the Gmaster for it's price. They should stop producing it, and make ASAP a sharper version (then i can have the 'luxury' but best in class price tag of 2000-2500€, AF also needs to be even faster, so that you have more reasons to pay for premium (see 135mm GM speed, a reason to buy it over Sigma 135mm art).
Overall: no contest: get the Sigma unless you are alergic to Vignetting (but tbh Gmaster has a lot too, just less). Do not buy Sony if you are allergic to chromatic aberration. Sharpess sigma is much better at infinity (think travel pics), sony a little better at close distance (think portraits). Noone of the things you expect to be bad on third party lenses, actually are bothering with the Sigma (focus speed, accuracy, etc). They are damn good.
The Sigma isn't perfect but for the price, and the class of lens its into (hardest to design lens zoom range) they did a remarkable job. Better then anything i've used to far. Could be still a bit close to prime sharp, but this is the first 24-70mm that i'm having no pain with to accept the Sony A7RIV (60 megapixel) images it puts out (the gmaster cannot follow this resolution). Sigma 14-24mm is even better (best zoom i ever used, having an amount of fun i never had with ultra wide angle on that lens).
Note to original post: you say Sigma has optical stabilization, that is not true, only DSLR version has.
I know for sure the cameralabs gmaster is sharper then mine (the one he used long ago on Sony A7RIV launch even and has no some sample pics in the 24-70mm review). Apartments are NOT that sharp except at 35-46mm (where the gmaster is prime sharp) but a good drop at 24mm and massive drop at 70mm.
