@TheProphet 

I think I can help you with that, but I need you to clarify few things for me first:

1. This is two floors house, the first floor plan is on the left of that image and second floor plan is on the right, correct?
2. What is the square footage of each floor?
3. What is the current speed of your Internet connection?
4. Can you run Ethernet cable from where your router is up to the Kitchen on the first floor?

  @Alexandre. 

Thanks for the reply. 

1. This is two floors house, the first floor plan is on the left of that image and second floor plan is on the right, correct? - Correct
2. What is the square footage of each floor? - About 1300
3. What is the current speed of your Internet connection? - 70 Mbps Download
4. Can you run Ethernet cable from where your router is up to the Kitchen on the first floor? - Not really, not without it being obvious and looking messy.

  @TheProphet 

I am going to start with basics of how WiFi mesh system operates, with specifics for your house. These are applicable to Deco and to any other brand of WiFi mesh.

1. Each WiFi mesh node has WiFi signal strength and coverage similar to those of standard WiFi router. That means, if you replace your router with WIFi mesh node, such as Main Deco, and it'll be exactly where your router currently is, your first floor WiFi coverage will not improve. 
2. WiFi mesh nodes need to have good communication with each other. They use standard WiFi protocols for that. Because of that, you shouldn't be placing WiFi mesh node in the left Bedroom of the second floor, your original plan. 
3. For the house of your size, it is recommended to place WiFi mesh nodes in the middle of each floor.

In summary, for a proper WiFi mesh deployment, you'll need Main node (Main Deco) where your Router is, and nodes in the middle of the first and second floor. Three in total. 

This would be recommended placement:

You will need to place Satellite Deco 1 somewhere in the Kitchen, and Satellite Deco 2 somewhere in the Landing area. Plus, Main Deco to replace Router or to be placed side by side with Router.

This will provide good coverage for both floors and ensure good communication between Deco nodes.

With your Internet speed, Deco M5 can do. There are two benefits for choosing M5 compared to X20:

1. It is easier to wall mount Deco M5, with third party mounting holder, which you may need to do in the Kitchen or in the Landing area. See Ceiling-mount/wall-mount suggestions of Deco. For the first wall mount option in that document all you need is an available electric outlet, no need to drill holes in the walls. 
    
2. With three Deco units, you would want Satellite Deco 2 connected to Satellite Deco 1, not to Main Deco. With Deco M5, there is beta firmware 1.6 which will allow you to make that configuration, and final code release is expected soon. See Deco M5 1.6.0. For X20, that feature is not available yet. With X20, Satellite Deco 2 may decide to connect to Main Deco, and that could degrade speed of your Internet connection for devices on the second floor.

-------------------------

One final recommendation. WIFi mesh of any brand is hit and miss. You may find Deco is perfect solution for you, but on this forum there are also people who can't make Deco mesh work for them whatever they tried. For that reason, it is strongly recommended to buy WiFi mesh hardware from the retailer with good return policy. You'll only need 2-3 weeks to figure if WiFi mesh you bought works for you. One month return policy is a must.

  @Alexandre. 

Wow, that was a brilliant reply. I can't thank you enough for the effort you put into that, I really appreciate it.

Is everything you said applicable to the M9 as well? I assume that would be a ebtter option than the M5 if I can find it at a decent price?

  @TheProphet 

Yes, what I said is applicable to M9, too. Deco M9 is better option than M5. It will provide stronger WiFi signal. You can wall mount it as easy as M5. Still, you will need three Deco nodes, even if they are M9.

I have tested M9 and M5 at my house, and between these two I prefer M9. Single M9 in the middle of second floor of my house provides excellent WiFi signal to all corners of that floor.

I would recommend to decide first, how many Deco units you will need. The only way to reduce your deployment to two Deco units is if you can run Ethernet cable from Router to Kitchen. That will make Deco unit near Router unnecessary. 

If you can't or won't do that, plan and budget for three Deco units.

Then, decide on form factor. If you need to wall mount Deco unit in the Kitchen or in the Landing, you can only choose between M5 and M9. They are sufficiently flat for that. Note that wall mount holders for M5 and M9 are different, due to different unit size. Make sure you order the right type of holder.

If you do not need to wall mount Deco units, you have more options than what you listed. These are Deco models to consider: X20, M5, M9, M4, S4, and E4.

Deco E4 will be the most budget-friendly, but it has 100Mbps Ethernet ports, not gigabit. Deco E4 is not recommended when Internet connection speed exceeds 100Mbps. You said yours is 70Mbps, as long as you don't plan to upgrade it in next few years and if wall mounting Deco is not necessary, check Deco E4 prices for 3-unit set.

@Alexandre. 
I have found good deals for both the M5 and the M9, is it worth spending more for the M9?
The M9 is about double the M5 price but still good value.

  @TheProphet 

M9 is more powerful unit than M5. Deco M9 will provide better WiFi coverage. Considering it is corners of your house that get weak WiFi signal now, it would make sense to spend more and buy Deco M9 Plus.

Also, if you expect to increase your home Internet speed to at least 300Mbps or more in the future, I would strongly recommend Deco M9.

One important caveat: you can't buy single Deco M9 Plus from TP-Link. M9s are sold in 2-unit and 3-unit sets. You must decide how many you need before you order, because if you need to add one more Deco M9 later, you may have to buy 2-unit set.

  @TheProphet 

This thread and question is not mine, but it is the best information I've had from ANYWHERE doing research on Deco. I have used deco m5 (4) in a large one-level home that worked excellently.l

Now I have a three level plus detached garage and need signal in the upper two living floors, basement, and in the garage.

This explanation has convinced me go to M9 and run ethernet from my T-Mobile Gateway on the upper floor facing the tower to our kitchen/living room area AND maybe to the basement!

Again this information is fantastic. thanks so much