Ups, sorry, I did not want to open the can of worms here. I only talked about my experience as someone asked on a post a couple of pages back.
Extremely low light venue, people moving around, not posed shots, the a7iii would not focus or would struggle, whilst the a9 did really well, it was a huge difference!
I did not buy the a9 because of the AF, but I was pleasantly surprised by it. The camera is so much quicker and snappier and an overall, to me, a better and faster user experience.
Obviously they are aimed for different purposes so one does not substitutes the other.
Original poster asked in relation to sports and that was my opinion based on that.
I have also used the a9 for landscapes since then, I would say the dynamic range in reality is more than enough.
Am I happy with a7iii? Of course I am, it is a great camera.
Am I OK having spent the extra money on a used a9 over a new a7iii? Yes, I do honestly think the price difference is well worth it iot just for the AF.
Which camera would I keep If I could only have one? The a9 now that I have it.
Thanks for that..... glad to know I have no idea of what I'm doing!