I debated whether or not to chime in on this topic. Mainly because as
@Steve pointed out technical differences are NOT a deciding factor between the two cameras. As demonstrated in the video there a few functional differences as to how some things are implemented that are a matter of preference. There's no point discussing the ergonomics because that's purely personal based on size of hands, personal preference, etc. After watching the video I pondered over which features of each camera I'd like to see in the other one. IOW what do I really miss when shooting one vs the other(ergonomics aside). All I could come up with still boil down to preferences. But here they are...
When shooting the Z9:
1) As a die hard full manual mode and ETTR shooter I REALLY miss not having the "zebra stripes" to indicate over exposed highlights real time in the VF. Z9 has the histogram but IMO it's about useless.
2) I miss not having the flexibility to program virtually any feature to any button. Z9 has significantly closed the gap in this regard but still annoyingly constrained on what function can be set to which buttons
When shooting the A1:
1) As a (mostly)manual shooter I miss the ability to change ISO with the control ring on the lens. Granted only works on S lenses.
2) I HATE the dials on top of the camera. This is about the only place that I depart from Steve's comments/opinions. In particular I prefer the way that exposure compensation is implemented in the Z9(with buttons set to "release button to use dial").
3) I much prefer the AF areas in the Z9 particularly since we can now custom design two wide areas.
4) I don't like DX mode. I really like how Nikon implements DX mode with the AF areas adjusting to relative size of frame and with AF performance improving(sometimes dramatically) with the subject larger in the frame.
At the end of the day anyone contemplating which route to take should be making a system decision not picking a camera. If I was "that guy" who only needs/wants one all around bird/wildlife kit with a single lens and as small/light as possible, you can't beat the A1/200-600 combo. For the widest array of lens options(f-mount inclusive) Nikon has the edge.
All said and done for me after a month of shooting them on alternating days and four continuous days shooting them side by side from daylight to dark, the Sony kit is for sale. It's great equipment. The Z9 and Nikon glass just work better for me.