@Alexandre. 
 

Appreciate the quick reply, many thanks!

What makes the X20 worse in terms of performance if we use ethernet backhaul? Is it purely the coverage?

  @NK123 

1. X20 is lowest model in X-series, except recently released X10 in some regions. X20 is budget model by price/performance. In general, nothing is wrong about it, but X50/55 is mid-range by price/performance. Should be better in general.

From the other hand, if you'll find X20s coverage is not that good, but you have plenty Ethernet ports per floor and can potentially place 2-3 X20s on Ethernet backhaul per floor, then having more WiFi nodes that are less expensive could make sense.

2. Judging from this forum, people have quite number of issues with X20s. Some of these could be from users themelves, but I don't see that many complains about X50/55.

3. X50/55 has three Ethernet ports instead of two with X20, you can have one more Ethernet port available to wire something to it.

4. Different Deco models have different feature set. Latest firmware for X20s is 2 years old, while X50/X55 get new features and have recent firmware update. Some of these features X50/55 have that X20 don't may be of importance to you, especially if you will need to expand WiFi coverage with more Deco units that use wireless backhaul. 

Check this document: Understand the Differences Among Deco Firmware Platforms Deco X20s will be on Firmware Platform 1.5, while X50/55 will be on 1.9.

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Most of it is subjective, but unless you plan to run professionally managed WiFi mesh supporting hundreds of devices connected to it, subjective could take over objective such as number of antennas and so on.

At my place I run WiFi 5 Deco mesh. If I had to replace it right now with WiFi6 mesh, I'll go with X50/X55. I would not even care of price difference between X20 and X55. Have you checked X50/X55 and X20 prices in your region? If you see same deal with X55s I have here in Canada and you plan to get Deco mesh soon, buy now. 

  @Alexandre. 
 

Many thanks for the extensive explanation! Sounds like X50 is the best option. The price difference is €60 between a X20 and X50

Might upgrade to 1 Gbit/s in the near future... I guess X50 would still offer better value than the XE75 right?

  @NK123 

You should be able to get gigabit WiFi speeds from WiFi6 Deco X50. Having XE75 with 6GHz wouldn't give you any benefit in speeds if you don't have strong 5GHz interference, and you said you don't.

Also, both X50 and XE75 have gigabit Ethernet ports, so even with 6GHz you won't exceed gigabit speed on XE75.

I suggest you double check X55 prices. Not sure what country you are buying them from, but in both Canada and UK 3-unit set of X55 currently goes for less than 3-unit set of X50s. As they are quite similar, go with X55s if its price is less.

There is also less hardware variances in X55s vs X50s. With TP-Link Deco, different hardware versions of the same Deco model will have different feature set. You would not know what specific hardware you've got until you have it delivered to you. X55 is safer bet if you want latest feature set.

Finally, make sure you get these from the place with good return policy. WiFi mesh is hit and miss, what works for someone else might not work for you. 1-2 weeks is usually enough to figure if what you've got is what you want.