I understand that 16-35 is much more $$$ and the 2.8 lets in twice as much light, also the components used are better and you get 1mm more
Can I assume that the shutter speed of the f2.8 would be half that of the f4? and with a tripod would the IQ be so different?
As other have explained, that is only true when shooting wide open.
What is true, and the reason why I bought att 2.8 lenses when I could - is THE VEWFINDER WILL BE TWICE AS BRIGHT with a 2.8 as with a 4.0
That is also important in low light conditions.
Good luck! Can you test and compare somewhere? How often will you use teh lens?
What camera are you using? I sold my wide lens, I rarely used it on a full frame. 24 was wide enough for 99% of my use on the 24-70.