@PapaStevie 

You can mix WiFi5 and WiFi6 Deco units in the same Deco mesh.

For mobile device, such as smartphone, that only supports WiFi5 or older WiFi 2.4GHz, there won't be any difference.

For stationary device, such as PC, there shouldn't be any difference. Makes sense to have that PC connected to Deco unit which supports WiFi6, of course.

For mobile device that supports WiFi6, you may see less agressive roaming between Deco nodes. That is because mobile device will have to make a decision: to stay with weak signal to the node supporting one WiFi protocol or to roam to the other node with stronger signal but different WiFi protocol. For example, should device stay connected to faster WiFi6 Deco node with weak signal, or roam to nearby Deco node with slower WiFi5 but stronger signal?

This is device specific, depending on decisions software developers made who wrote code for it.

It is also possible that you won't notice any difference. That, again, depends on mobile devices at your household and impossible to tell in advance.

Finally, when mixing different Deco units remember that resulting Deco mesh will only support features available for all units. For example, you can have "signal source selection" feature in Deco M5, but not in Deco X20 (as of today). If you have Deco mesh consisting of just Deco M5s and add even one X20 to it, that feature will dissapear from Deco app.

If you need specific features that are available in your Deco mesh, check if they are available for new Deco units you plan to add to your Deco mesh.