You asked
Sony fe 16 35mm f 2 8 gm ii vs tamron 20 40mm f 2 8 di iii vxd?
Both are f/2.8 zooms for full frame, but they cover different ground. The Sony FE 16-35mm f/2.8 GM II goes wider at the short end and is the one I'd pick for big skies and architecture. The Tamron 20-40mm f/2.8 Di III VXD starts 4mm tighter, so it feels more like a standard zoom. Its main advantage is mount range: it fits Sony E and Nikon Z, while the Sony only fits Sony E.
If you want the widest view of the three, the Tamron 16-30mm f/2.8 Di III VXD G2 is worth a look. It's a light ultra-wide zoom at 440 grams, and it doesn't extend as it zooms.
For landscapes, my general advice is in the landscape guide.
Written by Claude from this site's pages.