I loved my a9. The only thing I wanted it to have was more resolution, (so long as it wasn't at the cost of speed), and a fully articulating rear screen.
Not really sure why the screen is important to me as I hardly ever use the blooming thing. Just always felt a little cheated that a modern high end camera didn't have it but my trusty old Canon 60d did.
Not sure how they compare directly as my a9 went a few months before the a1 arrived. My gut says the a1 feels are 50% faster and is 40% more accurate, (read keepers).
In fact, I don't
think it has missed focus yet. Don't get me wrong I have had blurry images, but these were user error, camera shake/wrong shutter speed etc.
The main thing for me is the increase in resolution. The a1 is the highest res camera I've ever owned or used. It reminded me of going from dial-up to broadband, for those old enough to remember those days. The feeling of... "ah so this is what using the internet is meant to be like" And the increased res cost nothing speed was, quite the opposite.
But for £6.5k I still expected an articulating screen! (that I don't use anyway)
High ISO, I'll have to get back to you as I've had to push the ISO at all yet. Still not had much chance to get out with my camera which is getting annoying.