@Hagarno55 

There is no benefit of having different model for Main Deco. Deco M9 Plus is capable WiFi 5 device that can deliver WiFi speeds much faster than your Internet speed. I have two M9 Plus at my house, quite happy with them. 

The only problem, TP-Link in its wisdom does not sell single M9 Plus units, only 2-unit packs and 3-unit packs. 

I wonder how do you plan to connect barn Deco to the rest of Deco mesh in your house: by Ethernet or its WiFi? If by WiFi, have you tested if it'll work, with Deco M9 Plus you already have? Makes little sense to buy another Deco only to find that distance is too great for its WiFi reach to the house.

  @Alexandre. Thanks for commenting. The wifi actually reaches and works, but i need to get it inside the metal barn through the plastic 'skylights' on the side. Considered power line or using something lesser, say M3W. But from what I have read you lose some router settings/functions because of firmware compatibility. 

I knew there really wouldn't be much if any improvement. I just didn't know if there was some 'better' hardware for a main router since I was adding a node. Thanks again. 

  @Hagarno55 

If you considering something lesser for the barn, consider Deco M5. It is same family as M9 Plus, you will not lose Deco mesh settings and functions by adding M5. TP-Link sells Deco M5 in single units.

This is what I ended with. I needed one more unit for my workshop. I had Deco M9 Plus set, but can't buy single M9 Plus for workshop, so bought M5.

You can also wall mount Deco M5 with third party wall mount holder. That may give you more options with Deco unit placement.

Also, with the right pair of standard powerline adapters, you could connect M5 to your home through powerline. I tested M5 with set of powerline adapters I have and it established link to Deco mesh. This is a bit tricky, because not every powerline adapter brand and model is M5 compatible, but it is doable.