- more robust body (the changeable viewfinder of the F5 is a week spot, that is the reason why all manufacturers have abandoned changeable viewfinders)
- vertical grip optional = better flexibility
- improved metering
- better compatibility with AI and AI-S lenses
- more robust rubber-leathering
- i-TTL
- much better ergonomics
- no more this stupid security locks
- integrated data back
- more options of data recording
- better compatibility with MV-1
- better AF sensor indication in red
- better autofocus system
- more autofocus points, better positioning of these points
- much better dynamic autofocus
- better dampening of shutter and mirror
- much more silent operation
- better viewfinder
- better, bigger knobs, easier to operate in the cold with gloves
- more custom functions
- the control options using in vertical operation with the F5 are very limited; with the F6 you have much better control
- F6 is much more silent
- info monitor on the back
- EXIF data visible on the back monitor, not only in the stored data (via MV-1)
- more refined mechanics
- high speed TTL / TTL FP with SB-800
- Func button
..................and even some more improvements.
Have a look also here
https://filmbodies.com/cameras/camera-reviews/nikon-f6-review.html
and here
https://f6project.com/