@Carvendive , The current latest Mesh unit is the M9.

I have 3 of these M9s in a 3.5k sq foot 2 level home and love it. They just work, flawlessly. 

I think your decision should be guided by how much you want to future-proof your network. Within the next 1-2 months, TP-Link is coming out with the next generation Mesh routers, the Deco AX10 ... these are compliant with 802.11ax standard, or wifi 6. Note: wifi 6 requires both the network appliance and the client device (cell phone, lap top, etc.) both be compliant with .ax. These new Mesh units are expected to be backwards compatible with 802.11 AC.

With your current ISP speeds, you will likely not notice much difference between the M9 and the X10s. However, the x10s should have a larger coverage area, so you may only need 2 of these where, like me, you may need 3 M9s for full 100% coverage in your home. It depends on the home configuration and where you place the devices in the home. 

Personally, I think spending a little up front to future proof my network is a good idea. You may only have 50MBPS today, but in a year or two or three, maybe you'll have an in-home 5G option, or fiber... or maybe you'll have 50 network devices in 3 years... all good reasons to have the X10 now...

If you're just trying to meet your current need, you could probably get by with the more innexpensive models.

:) John

@Carvendive this table comparison is also very helpful:

What are the differences between different models of Deco?

https://www.tp-link.com/us/support/faq/2423/

@JohnBruso 

Thanks John.

After posting my original question I was able to find a resource on tp-link that helped answer the "which one to buy" question. For me the answer came from my ioT requirement. M9 it is (or was) until you mentioned the upcoming release.

My last router (a Cradle point MBR-1000) is over 10 yrs old, so yes, future proofing is on my radar. And you're correct. Who knows what'll happen once that new satellite system is up and running.

For anyone else watching this thread, this TP-Link post looks very helpful as well: Similarity and Differences of Deco Models https://community.tp-link.com/us/home/stories/detail/768

more info about .ax/wifi 6 (only available in the Deco AX10 and higher - Coming Soon): https://www.tp-link.com/us/wifi6/

@JohnBruso 

Well... I couldn't hold out for the new release after all. I got a 3 pack of M9's. Costco is to blame! $199 for the three pack and free two day shipping.