When comparing Panasonic Lumix S5 IIX vs Nikon Z6, the Slant community recommends Panasonic Lumix S5 IIX for most people. In the question“What are the best professional mirrorless cameras under $3000?” Panasonic Lumix S5 IIX is ranked 6th while Nikon Z6 is ranked 9th. The most important reason people chose Panasonic Lumix S5 IIX is:
Good video quality, features and performance.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Video quality
Good video quality, features and performance.
Pro SSD recording
Can write directly to external SSD devices (up to 2TB) over its USB-C socket.
Pro Good stablization
Pro High-ISO quality
Superb for low light conditions.
Pro XQD card slot
supports XQD 2.0 and 1.0 cards, see Nikon Z6 XQD Card Comparison.
Pro Full frame 4K video
Pro In-body image stabilization
Cons
Con Sinister black look
A question of taste of course, it seems to be an "award-winning design".
Con No tally lights
Con Not all reviews are positive
Here is a critical review: BEST MIRRORLESS CAMERAS OF 2019.
Con Display tilt
The display can not be tilted 180 degrees for selfies.
Con Buffer capacity
Average buffer capacity which slows down performance especially when using older and slower XQD cards.