I'm debating right now between the 70-200 f/4 and the 70-200 f/2.8 for my A7III while i wait on my check to get here
I dont mind the weight, though smaller is nice, i used the Canon 70-200 f/2.8 II on my 7DII for a long time and I had the f/4 before that. Isolation wise the f/4 will be just like the 7D and f/2.8 after all.
My biggest issue on the f/4 is its MM is TERRIBLE at 1:7.6 versus 1:4 on the GM or 1:5 on the Canon..heck its worse than my 55 f/1.8 Sonnar's 1:7...Yes I have the 90mm macro, but sometimes i need that combination of reach and ive found the 1:5 or so MM to be a good "spot" for me
I use it for a lot of things, im a heavy telephoto shooter and 70-200 is a heavily used lens in my applications, one of which is using it for larger flower specimens (or flowers where i simply just cant get the angle or composition i want or i need the background compression to destroy a distracting background) and also action shots of my two Cardigan Welsh Corgis, fast AF is a must but both lenses sound great in that regard... So it basically comes down to IQ and the MM
The biggest issue on the GM is i guess i feel im paying a LOT just to get the better MM, i could easily get another lens i dearly want (the 24mm f/1.4 GM) for the money TBH, but at the same time id hate to buy something i find out isnt going to work out well for some of my photography (Specifically some im not going to be able to test until spring) and then having to go spend the money later on the f/2.8 i should have bought in the first place