@danysanz 

Hi, welcome to the community.

It might depend on what you want to get from WiFi 6. 

If the current interbet bandwidth is about 800m-1G, and more mobile clients start supporting wifi 6, Deco X20 would be a perfect choice.

But if the current devices are more wifi 5 and the internet bandwidth is also below 500m/s, adding one more Deco M5 would be more recommended.

Since all Decos work with each other, a mixed system with Deco X20 as the main Deco, M5 as satellites, the wifi 6 devices could also enjoy the wifi 6 benefits when they are connected to Deco X20.

And you could also refer to this link to see the difference between Deco M5 and Deco X20:

https://www.tp-link.com/en/compare/?typeId=5700&productIds=58738%2C42880

Thank you very much.

best regards.