The best hardware is usually the hardware you already have. Either machine will be perfectly adequate for getting started. Don’t let analysis paralysis stop your progress. Pick one and run with it. Just keep in mind that when/if you run out of resources or find you aren’t using many resources you can always move. So write scripts, take notes, and think about backup and restore from day one.
I personally would start out with the RPi 3B which should be adequate assuming your voice detection stuff doesn’t need too much RAM. If you do something with the AIY Voice kit and work with the hardware directly instead of through Google’s APIs some of that processing can be offloaded to the hardware hat instead of the CPU.
Given that your voice detection is TBD, it is pretty much impossible for us to say whether the RPi would be sufficient. It’s plenty sufficient for running OH and Mosquitto and a number of other services. But you don’t yet know what the voice stuff will require. I think some are running the software I recommend below on the same RPi as OH.
From a hardware perspective, you will have more flexibility on the RPi. But you already have a screaming desktop not doing anything so you might just start there and know it has enough power to do what you want. But it might be way overkill and once you are done it will be sitting there sucking down your electricity not doing much.
There are tons. Look into Jarvis, Jasper, and Kalliope which would get you started (note they all run on RPis).
If Windows updates is your big issue with Windows then you should know that most Linux distros work on a pretty much continuous upgrade cycle. There is hardly a day that goes by that running an update doesn’t have some library or program that has an update. But unlike with Windows, the update checks pretty much ALL the software installed, not just the OS. I do not recommend skipping updates.
Despite this, I run all my servers on Linux and I find it to be very stable. I haven’t turned on my last remaining Windows machine (a 5 year old laptop with a busted hinge) since I got my Chromebook last December.
I’d recommend openHABian there too. openHABian is just a bunch of scripts to install and configure OH and related services on any Linux that supports apt. Just follow the manual instructions and you can have all the goodness of openHABian on the ODroid, an Ubuntu VM, etc.
Snips is a new one to me. I’ll have to look into it some more, though it’s mainly out of curiosity. We’ve become a Google house and are pretty happy with the Google Homes for this job.
That’s the real job though, isn’t it. And that’s where the fun lives. The choice in hardware really doesn’t matter to much in solving this problem. And frankly you don’t know enough right now to really know what the optimal hardware needs to be. So I still recommend just getting started with the hardware you have. As you learn more you will discover if you need to move later.
So your problem here is going to be how do you wire up all those microphones to one machine? If you have six rooms with six microphones you need six microphone ports on your centralized machine. And most decent voice control systems require multiple microphones per room. A Google Home has something like eight.
That’s a lot of sound cards and a lot of wiring to run. There are some very compelling reasons from a hardware perspective to consider a separate SBC in each room. You might be able to do all the processing centrally, though personally I’d rather have as much processing be done locally to the room instead of centralized. It really isn’t that much more work to deploy more than one nearly identically configured device, particularly if you automate the configuration (I like Ansible for this).
I’ll provide my standard advice that comes from a Mitch Hedberg joke.
An escalator can never break: it can only become stairs. You should never see an Escalator Temporarily Out Of Order sign, just Escalator Temporarily Stairs. Sorry for the convenience.
As you plan and design your home automation, consider the failure cases. Make sure things fail gracefully into a perhaps less convenient but still usable state. Strive to build escalators instead of elevators.