@hakmusic 

I would actually go for the Deco M9 due to the M9 having the wireless backhaul. Compared to the M5 and P9 the M9 has a bit more coverage. 

If you opt for the P9, the power adapter would need to connect directly to the wall outlet, using a surge protector will interference with the powerline signal so the P9 will end up utilizing Wi-Fi.

The M5 is definitely more affordable compared to the M9 so there is more flexibility in placing your nodes in three places rather than two.

You can have up to 10 Deco units, so you can start with a three-pack, and add as needed.

The M5 is available in 1/2/3 packs, M9 is 1/2, and the P9 is only 3.

@Tony   Thanks for your response.  So you think, say, three m9's would be more effective then, say, five m5's because of the dynamic backhaul?  Picking those numbers based on relative cost.   If the backhaul makes a noticable impact, it seems like that would make sense (i didn't see the m9 on amazon originally, but now i do). 

@Tony   One other question - would it make sense to get 3 pack of the M9, use it as the base, and then get a 2 pack of the M5 to extend the system.  Would I lose the advantage of the dynamic backhaul if I did that?    thanks

@hakmusic 

I supposed having 5 M5s nodes does sound better in terms of coverage.

If you were to have the M9 as the main, and satellite M5 nodes, there would be no wireless backhaul.

In the end, it is difficult to really determine what works best other than actually testing. So I would say go with the M5's, but make sure you can return them if needed, and monitor them, see if they perform at the level you need. If not, you have the M9s as an alternative.

@hakmusic I'm a fan of PowerLine for when you can't run an ethernet cable.  I was using it when wifi wasn't that good to put my main computer on the second floor onto a wired connection, or when a Roku couldn't get a strong enough wifi connection.  As long as the house wiring isn't too bad you should get decent speeds.  I'm renting the 1st floor of an older house so I'm guessing the electrical wiring is at best just OK.  From the satellite P9 connected over PowerLine my iPhone 8 speedtest hit 80 Mbps.

I bought the P9s specifically because I had the option of ethernet, powerline, and wifi for the back haul.  Iirc, the TP-Link marketing materials show a picture of a 6000 sq ft house with a P9 on each floor.  I'm only using 2 P9s for about 1200 sq ft on one floor and it is more than enough.  I've got thick walls so one wifi device wasn't enough, and because of the thick walls I didn't think a wifi backhaul to a 2nd AP would work well, so I went with the PowerLine backhaul.