While one has the X1 Extreme chip the other has the X1 Ultimate, which could be where some of the difference lies. I'll also say that there may be more to the cost difference than simply image quality. In my experience, from Sony's lower end to mid tier to higher end, there can be a pretty big difference in BUILD quality. Their higher end TVs tend to have lots of beautiful brushed metal, heavier and built more like tanks, and lots of little details that won't come through in a review or just looking at them side by side. I've installed a lot of Sonys over the years, and every time I'd install something like a Z9D, I couldn't help but walk away really, really impressed by the "feel" of the TV. When you handle them like I have, getting intimately familiar with the backs and sides of them, that's when you can really appreciate some of those differences in construction. If I'm not mistaken, the A8F is kind of their "lower end" OLED while the A9F is out of their new "MASTER Series" televisions, ie higher end. Most likely the same exact physical screen, but I'm sure there's a difference in image processing and overall build.
Whether or not that cost difference is "worth it" is entirely up to you. There are ALWAYS the laws of diminishing returns at play for sure, but that's the way it's always been. Some people simply want the best. Even if it means paying a much larger margin for a much smaller gain. And they're ok with that. Some people demand the best performance:dollar ratio, in which case a 900F is a solid buy as that seems to hit the sweet spot before you start paying a lot more for marginal improvements.
