I think this would be a perfect opportunity to visit a store and check both out.
In my particular case the Tamron 17-28mm came out after I purchased the GM16-35mm. Since it did not exist at the time I was in the market there was really only the native GM16-35 f/2.8 for a fast f/2.8 option.
Questions you should ask yourself is if 28mm is long enough for you for other purposes. 17mm is only 1mm difference than 16mm but some may find an issue with that.
Once you hand hold the 17-28mm and want small form factor you may just be over the moon in saving $1300 USD vs buying the GM16-35mm. The Tammy 17-28mm is tiny!!!
I use my UWA zoom lens for general purpose. 35mm is extremely important and is worth for me to go with the GM.
In post processing I think in many cases IQ wise the Tammy vs GM can be equalized and almost on par. The thing about lenses is we all may have some sort of preference to how the glass renders. For example my recent purchase of the Sigma 24-70mm ART zoom, it seems to render different to all of my Sony dedicated glass. The colour and render is much more pleasant straight out of camera vs even my Gmaster 16-35mm or Tamron 28-75mm.
If I wasn't so stuck on requiring 35mm on the long end my "sensibility" would push me to go Tamron and use the extra cash on other lenses to supplement the gear bag.
Thanks for your perspective! Since picking up the 105Art, I've been toying with the idea of selling the magnificent 50 Art in favor of a wider range zoom. Currently using an adapted Rokinon 14mm, the 50Art, and the 105Art, so there are a lot of gaps in focal length. I could probably get by with either a 16-35 or 24-70 to fill a lot of that. I tend to enjoy using primes more, but I do find myself a little limited with my current setup. I will never part with this 105!