I strongly favor the 800mm pf. I find it to be a remarkably sharp and adept lens. It is permanently mounted on one of my Z9 bodies. I find it tracks birds in flight very well when using a wide focus frame and bird subject ID. It is my lens of choice when I go out to shoot birds.
I rely on the following combination of lenses to cover my telephoto needs:
1. 800mm pf
2. 400mm f4.5
3. 70-200mm f2.8.
I prefer to work with high IQ lenses because they work better with cropping/dx to gain reach. I almost never use TC's with this lens combination.
The only thing that might tempt me away from th 800mm pf would be the 600mm f4 tc.
With the 800mm pf I stringy recommend you use the OEM case that comes with this lens.
f you get one of these used I recommend you make sure to get the original carrying case that comes with it. The large front element on this lens is potentially vulnerable. The original case does a really excellent job of protection. The case will hold the lens with camera attached.
For my 800PF I also have a lens cap from a German brand. In the US everyone seems to use Zemlin. The Nikon cover for the front-element of the 800PF is a bit annoying and a lot of fumbling around with the cord, especially if you have to be really fast.
The 600TC has a much better cover, I think Nikon should have used a similar one for the 800PF. It's much faster to attach and remove. I have not purchased a separate lens cap for at as the German brand does not have one for the 600TC. I will probably end up buying one from Zemlin.
Do note that you will also need a separate lens foot for mounting it in a tripod, at least if you have an arca-style mount for your tripod head. It's really annoying that Nikon does not ship an arca-compatible lens foot with their expensive glass.
The 600PF has a very solid lens cap from Nikon, it's just 95mm, so nothing special is needed for that one.
Optically I find all three of these lenses to extremely sharp if the condition is righth (no heat haze, fast enought shutter speed)