@Brewjam 

My house is about the same size, brick exterior, wood interior, two floors and finished basement. I have under 10 mobile devices at any time connected to WiFi, and half of them are low bandwidth, such as ecobee thermostat. My ISP provides me with 300-350Mbps Internet. 

Before Deco I had ISP router providing WiFi and 5GHz was not getting far, 2.4GHz was weak in some places of my house. It is with ideal placement of the router: in the middle of the first floor.

With Deco, all my house is covered with strong 5GHz WiFi signal. It is so good, I disabled 2.4GHz on Main network that Deco runs.

I suggest you go with 3-pack and put Deco M9 on each floor and in the basement. Initially, I went with two-pack, for first and second floor, but found that I need third unit for basement to make WiFi coverage good everywhere. 

If you read marketing materials, they may imply that for the house of your size 3-pack is overkill. I would not trust marketing materials on that topic. Especially, if you want 5GHz coverage.

I suggest going with M9. That is what I have, and it works very well. I am getting 350Mbps in close proximity to wired Deco M9, which is max ISP Internet speed,  and 300-350Mbps near Satellite wirelessly connected to Main Deco. You should expect 500Mbps in similar conditions near wired Decos and 300-500 near those using WiFi backhaul. 

I do agree that gigabit is an overkill for WiFi devices in average household.

If you ever need it, you could wall mount or ceiling mount M9, by buying mount holder for Deco M9 from Amazon. 

Doorbells are pain in the neck, according to posts on this forum, but you could create separate Guest network that only runs on 2.4GHz and connect doorbell to it. If you plan changing it, it makes sense deploying WiFi mesh first (even if not Deco) and after that look for compatible doorbell. If new doorbell does not work with WiFi mesh, it would be easier to find different brand of doorbell than to replace mesh system.

Your ideal deployment for a house of that size and that type, which is just like mine, is to put Deco units in the middle of each floor. That is a bit contrary to what Deco documentation suggests, but Deco node sitting in the middle of the floor will provide good coverage for that floor, and will have good WiFi backhaul with Deco unit right above or right below it.

If you can add Ethernet to second floor, it is always better than using WiFi backhaul, but with Deco M9 correctly deployed you might find you don't need it. Yet, it is an option for Deco units.

Ideally, your Main Deco should be on the first floor, wired to router. 

@Alexandre. So you think the improved coverage of the m9 would be good enough? I do wonder if I am wasting the 1Gbps by not getting the x20. 
 

also I have a desktop computer hardwired in the front of my house. I would just need to run another cat6 to it correct?

@Brewjam 

As long as you have at least one of Deco nodes not wired, M9 could be better solution. Its dedicated backhaul will make the difference.

If you plan to wire all Deco nodes, which means you won't need WiFi backhaul, X20 could be better option. It supports WiFi6, which at least on paper gives faster speeds. You will need mobile device capable of utilizing it, of course.

Non-scientific tests in my household, with ISP speed at up to 350Mbps show the following:

Main Deco M9 wired - 350Mbps

Satellite Deco M9 wireless, using dedicated backhaul - 350Mbps

Satellite Deco M5 wired - 350 Mbps

Satellite Deco M5 wireless, does not have dedicated backhaul - 170-200Mbps

As you can see, for Deco units without dedicated WiFi backhaul, such as M5 and X20, a substantial speed loss should be expected if Deco unit is not wired. Which could very much negate X20 performance vs. M9.

Then, assuming wireless X20 is not better than M9 in that respect, perhaps same or a bit worse, the decision comes to price point, form factor, and WiFi6 support. It will be up to you to decide.

As for me, when I was choosing Deco mesh, at that specific time 2-pack Deco M9 was substantially cheaper than 2-pack X20 on Amazon in my region, I needed unit that I can wall mount in narrow corridor, and WiFi6 support is not relevant for me.

>I have a desktop computer hardwired in the front of my house. I would just need to run another cat6 to it correct?

Yes, it is correct.