@JonR 

You need to be more specific talking about "performance improvements" you expect. Pick one:

1. Elimination of dead spots;

2. Download speed improvement when connected to Satellite Deco;

3. Download speed improvement when connected to Main Deco.

Perhaps, none of the above, but something else? 

@Alexandre. 

Maybe these two are my main concerns:

1. Elimination of dead spots;

2. Download speed improvement when connected to Satellite Deco

You might want to include there the latency and if the elimination of the need for a wired connection (I want everything to be wireless as much as possible). Basically, I want to know which among the 2 models (M9 Plus or X20) will I benefit the most given the limitations of my M5's. What are the limitations of the M5 by the way when mixing with the P9 plus and/or X20?

Sorry for multple questions. Just want to come up with a well thought of decision.

Thanks!

@JonR 

I also forgot to ask you what is the speed of your Internet connection. If it is under 500Mbit now and will be in the next 5 years - no need to spend on WiFi 6 Deco equipment such as X20, X60 or X90.

If your Internet speed is 1Gbit or above, makes sense to replace main node with WiFi 6 capable node, as a future proofing network.

Internet speeds in between 500 Mbit and 1Gbit - comes to personal preferences.

Except, I suggest to skip X20. By specs it is not much better than M9 Plus, yet at least in Canada where I live, right now I can buy two M9 for the price of one X20. So, if you have high Internet speed and desire to utilize WiFi 6 - consider X60 instead. More expensive, but much better. Alternatively, just wait till good deal on X60. 

Unless you want to put your money to better use - then, buy two M9 Plus.

In regard to your current network. Replacing Main node will not improve it, because overall mesh will be as good as it is now. Satellite M5 will not be transferring data faster if Main is M9 or X20/X60, dead spots around M5 will not improve. 

The only benefit you might see in replacing Main Deco with X20 or M9, and only with Internet faster than 500Mbit and your mobile device supporting WiFi 6, is near the Main Deco. If you have dead spots near Main Deco, or you sit in front of it while downloading huge files from the Internet, then you may see benefit from upgrading.

What I am saying is you might need to focus on eliminating dead spots by different means, such as moving Satellite M5 around if possible to find better location for them, or if you can hardwire one M5 with another, perhaps buying one more M5 for specific dead spot and wiring it back to the nearest M5 you already have.

You can mix different Decos, no problem, but in general, to get most benefits of different Decos, you'll need them in pairs of the same type. For example, I have a pair of M9 Plus, with good 5GHz signal between each other. They benefit from having good 5GHz backhaul on a separate channel, which is feature for M9 Plus specifically. They sit in the middle of the first and second floor of my house, covering both floors with 5GHz signal nicely. My download speed on tablet running on 5GHz in the far corner of the second floor is as good as maximum Internet speed from my provider, which is 300Mbit.

I also bought M5 for the far corner of my house basement where my office is, that Deco is wired - and this is how I eliminated the remaining place with weak signal, where it matters.

Hopefully it helps.

@Alexandre. 

Thank you for taking time to answer my concern and yes it was quite comprehensive. But it actually makes a lot of sense. I forgot to tell u that my internet service is 100mbps. Also, I have 2 additional M5's that got from my service provider. I guess for now the best way to go is to hook up all 5 M5's, then upgrade to 2 M9's later on and use 2 M5's as satellite nodes. 

Thanks!