Image quality test results have been added to the Sony FE 70-300mm f/4.5-5.6 G OSS Lens page.
Here are some comparisons:
Sony 70-300 vs. Sony 100-400
Sony vs. Canon
Sony vs. Nikon (different resolution cameras)
Sony 70-300 vs. Sigma 100-400
Sony vs. Tamron
The Sony FE 70-300mm f/4.5-5.6 G OSS Lens is in stock at B&H | Amazon | Adorama | WEX.
Rent the Sony FE 70-300mm f/4.5-5.6 G OSS Lens from Lensrentals.