Great TV and great value, but the A9 is the ultimate 4K HDR OLED TV. Hope you don; tmind my adding some of the differences between Sony's A8F and Sony's new A9F flagship Master Series OLED TV.
The A9F has better motion resolution and better up-conversion of FHD/SDR and even better low luminance gradation detail. In all fairness the three video advantages are all moderate, but when consider all together they add up to a very nice moderate picture quality upgrade.
Another A9F advantage is it's built-in 3.2 channel audio system that has front L/R/C and two subs, the A8F has a 2.2 speaker system so it lacks the clear dialog and all vocal content that the A9F's dedicated center channel audio delivers. The final extra feature of the A9F is that you can use all three front channel speakers as one center channel when connected to any surround sound receiver.
It's a tough and personal decision for anyone looking to buy a premium TV as you are looking at two excellent TVs, the price/performance best choice with the A8F or go for the new King of TV, A9F.