I have installed three 8870 Thermostats to my M1. The setup I have, has one 8870 controling the second floor heating and AC, a second 9970 controls heating and AC for the first floor and the third only controls heating for the basement.
All fun, however, there are a couple of things you might want to think about before making the $$$$$ plung.
It is recommended and I did install:
(1) 8818 distribution panel, makes support and trouble shooting easier, or so they say, not to mention the panel has built in protection (fuses) to protect your very $$$$ thermostats.
(1) 8811 Protocol adapter
(1) ELK-M1XSP Lighting/Thermostat Interface , Serial Port Expander. which you need if you already have the db9 serial port used up as I did with the ELK-MK1XE.
Last but not least $$ (3) 8870.
So you really need to be aware of the complete picture, before you make your choice.
Now one thing that not many people look at is, what kind of support do you get when you NEED it? While not many people look at this, at some point you will need to place a call for support; you really don't want to go into a LONG que or send an email and WAIT... or dealing with BAD to non-existant support.
My experiance with APRILAIR, is that their support is OUTSTANDING.... They really know their stuff... that is what I need when I need it... P.S. did I mention that I got right through.
Also, again I have only worked with AprilAir.. Well made construction, quite flexible and they do get the attention of the AC and heating folks (WOW)...
Looks well that to me is personal, I liked the AprilAir, they appear simple yet nice, not a retro looking panel which you and all will see...
Hope this helps..