Nits is the brightness of image, HDR images can track up to 4000 nits but different brands have different ways of handling it (Sony appear to track natively up to the TV brightness then map higher brightness down, so the higher the better). Check out the videos on the HDTVTest YouTube Channel (he also has a 9005).
I must admit that coming from a 2017 Panny VA panel the difference is less severe on the angles than the Sony VA panel (something I only really noticed today - it's there but just less).
If you're thinking about spending the coin for the
9000H, then I'd just go for the 9505 TBH (
55 gets better tech, but the
49 scores really well in reviews which I've linked to in this post).