rtings has a review up on the new M series. Although the overall score is about the same, I think that the 2017 M is a superior display for everything that has to do with HDR and watching movies.
The majority of score-hits it takes has to do with the fact it's a 60Hz panel and that there's no more low-latency port.
It has much better brightness for highlights (possibly brighter than the P for small highlights) and it has wide color gamut (same as P series).
But it's down to 32 led zones, it doesn't have HDMI 5 low latency port, and all models come with a 60Hz panel (it still does 24P properly though).
So it's a bit of a trade off. I'm thinking about upgrading my M55-C2 (2015) to the new model.
Judging by my M55-C2, which also has 32 zones, the reduced amount of zones is only really a 'problem' when you have a bright logo on a dark background, and you're watching in the dark. (there's some blooming around it)
In a bright room or content without any bright logos, it still looks great.