@DevonAlex 

My Deco mesh is two M9s and two M5s. Main Deco: M9. I have this set for almost two years and they work together just fine.

Main Deco M9 provides faster WiFi speeds than Main Deco M5. I tested that, and I am assuming it is because of stronger WiFi signal. It is recommended in M5/M9 mix to have Deco M9 as Main Deco, so you do plan it right.

For the pair of M9s, you'll get the most benefits if they connect wirelessly to each other. In any case they will be at least as good as M5s, but if your Satellite M9 is wirelessly connected to Main M9 and they are in proper distance from each other, wireless devices connected to Satellite M9 will get faster speeds.

If all your Satellites are Ethernet wired, it is less of a difference, but wired M9 will still provide faster WiFi speeds than wired M5.

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These are speed test results from my Deco mesh set, for the reference. These are the best results I've got after dozens and dozens of speed tests. If you get, say, 10% less speed from your M5 than I am from mine, expect about 10% less from your (future) M9 comparing to mine.

Main Deco M9: 635Mbps

Main Deco M5: 475Mbps

Satellite M9 (wirelessly connected to Main Deco M9): 490Mbps

Satellite M5 (wirelessly connected to Main Deco M9): 225Mbps

My Internet connection is gigabit.

Very many thanks for confirmation

Alex