Here are my general thoughts based on the Nikon Z9 + 600mm TC rental vs my Sony A1 + Sony 600mm GM rental. I had the Nikon for a week (in lousy weather) and the Sony 5 1/2 days (in beautiful lighting). These are just my personal opinions, and as many say here YMMV.
LENSES
- VR - Nikon outstanding; Sony ok.
- Lens foot - Sony slightly longer so slightly more comfortable for handholding for someone who has to attach the foot to their BR strap connector and for holding while walking.
- Hand-holdability - about the same, but surprisingly even though the Nikon is heavier, it is shorter so I found it easier to maneuver when photographing from low positions or from moving docks.
- Built-in tele - definitely a plus for the work I do.
- Customizable lens buttons are reachable by both the thumb and index finger on the Sony but I really had to contort my hand in order to reach one button on the Nikon.
CAMERAS
- EVF - Sony almost "real like" and I would give them the edge with their blinkies; Nikon workable but not "real like" although the workaround for blinkies is very helpful in the field.
- AF - Nikon is reliable and stickier even in changing backgrounds. On Sony I can only use Zone or spot, but the Nikon's Auto (similar to Sony's Wide) was so outstanding I could use that for diving birds even in low light situations. Nikon's Auto stays on the bird better than my A1 even around water. It was amazingly reliable and easier to use! Nikon also did a fantastic job finding the eye in difficulty situations. Also, 0 issues finding/tracking a white bird.
- Buffer - on the Z9 I had no buffer anxiety including when spraying and praying during testing. On my A1, I often play the "wait wait wait" game and hope that I have enough buffer left.
- Grip - Nikon was more natural and more comfortable for me; on Sony, I always use the battery grip and have to overextend my pinky into the VG to hold the camera.
- Button customizations - I found the Nikon Z9 to have all the customizations I needed similar to my Sony A1. That said, I do wish it had more buttons on the back of the camera (e.g., AEL button like Sony) that could be customized since I had issues reaching all of the Fn buttons on the front of the Z9.
IMAGE QUALITY - the jury is still out. I have fallen in love with the Sony images the past 2.5 years and loved the attempted fishing views I shared here with the 600mm GM rental. I spent many hours trying/hoping for a similar opportunity when I had the Nikon rental but it wasn't meant to be. This is a huge decision for me given the amount of money. I also wish the EVF was more like Sony's and really don't want to wait over a year for a Z9ii. Perhaps it is best to purchase a used Z9 in the meantime while I wait for the Z9ii. I'll need time to thoroughly review images before deciding.
Thank you all again for your support and help!! Very very much appreciated!!