Hello!
I have a Pi 3B+ or a Pi 4 4GB available for usage with Hass.io/Home Assistant.
Which one would you choose for it?
The Pi 4 is faster and has more memory so the experience would be better,
But the Pi 3B+ can be used from an USB drive alone so no SD-Card is involved.
In my experience, definitely go with the Pi 4. It is almost as fast as my small NUC. Get an industrial SD card if you have experienced issues.
I’ve been running HA on my (then brand-new) RPi 3B+ for about a year now. Never had a single problem with performance. I use Hass.IO and haven’t loaded it up with a bunch of video or anything really intensive. I haven’t added other applications, just HA.
So I’d say it all depends on what you plan to do with it. Personally, I’d use the oldest/slowest Pi for HA, saving the newer/faster one for my next project, where it might be needed. You can always swap them out later if you find you’re unhappy with the performance.
I moved from a 3B+ to a Pi4 and it’s like night and day. The 3B+ was running off an SD card and the Pi4 is running off an SSD via USB3 with an SD card for booting. Updating the 3B+ could take 20 minutes or so to finish and the Pi4 is done in 5 minutes or less and restarts take a minute or less on the Pi4.
I have the 4GB version and have it overclocked to 1.8GHZ at the moment but it does run very well and would recommend the Pi4 with an SSD.
Hard figures show it best and after moving over to the Pi4 I moved my 3B+ over to SSD boot as well and this is a throughput comparison showing you real world speeds - Pi4 at the top and this is with a cheap USB3 enclosure and cheap 120GB SSD.
Is there anywhere an installation guide availabe to get this running?
I think the 120GB SSD is nearly empty, because ther is surely not so much data necessary for Home Assistant. I think I will try to get a smaller one. 
Thank you all for your answers!
I moved 3B to 4B 4GB. The 3GB extra RAM allowed me to put whole of Addons and Homeassistant folder into RAMdisk. This then will minimise problems on the microSD card.
The speed increase is also a bonus.
You say you have the two Pi’s. Is there other application that you need a quicker, more RAM Pi?
I have a 3B+ running with a Raspbian/homebridge installation.
And I have the 4B 4GB with a testinstallation of Hass.IO/Home Assistant. I am playing with all the features and things I can do with it.
Afterwards I will only have one Hass.IO installation as I think homebridge is not the solution I want, because all my automations are running in homekit where I have no backup of them and if the icloud is not available all automations are not working. So I think that Home Assistant is the better solution. Only the configuration is not really easy, because I don’t know really where to start and where to find something. That is handled easier in homekit. There you have an event, conditions and something to switch on/off or whatever. Everything you can choose from lists.
The SSD was about £20 so really cheap and branded. As it’s a Raspian Buster install I can use the space on there to install and run other programs if I wanted.
There is a Pi4 guide on here but I had to follow this Guide to add Quirks to get my enclosure to boot the Pi properly. `
I searched in the meantime and 120GB seems to be the smallest size with the cheapest price. I saw WD Green and Crucial BX500 for around 23€.
Then I will search for the guide here because I don’t want to use Rasbian.
The 2,5" SATA to SSD adapters I have are working with the Pi 4 without a problem. I have two other M.2 SATA SSD adapters that don’t work properly on the USB 3 ports of the Pi 4. They are working without any problem on the Pi 3B+.
Why don’t you want to use Raspian? It gives you full SSH control over the Pi and if you want to stay with Hassio you can still install the SSH add on and use it via that only.
Not sure how well the SSD boot of Hassio is coming on yet but with a SSD of that size it seems a shame to not be able to use the rest of the drive and the Pi for other things.
I have the BX500 SSD BTW 
I wanted to install Home Assistant with the underlying Hass.IO because I don’t want to care about updating the system on my own every time. Or is it possible to install Hass.IO on top of Raspbian?
Yes you can, install it in docker.
Home Assistant (formerly Hass.IO) is Home Assistant in a docker running on Home Assistant OS (formerly HassOS).
Installing HA (hassIO) in Docker on Linux doesn’t actually get you the full HA (hassIO) user experience. This install method is now considered “Home Assistant Supervised” (formerly “Hass.IO Generic linux install method”).
The biggest feature missing with Supervised install is automatic update of underlying OS, which I gather it is an feature OP wants.
Yes, correct! I don’t want to manage two “systems”.
But I think it is not possible until now with a Pi 4. So I stay with an SD-card and will be happy if the Pi 4 will work with “normal” USB boot in the future.
There is no managing of Hassio in docker using the generic install - I just hit the same update button that I did under HassOS on SD card and it updates to the latest version.
The base OS of Raspian is only a couple of commands to get the latest updates.
If you want to use the full speed of the Pi4 then SSD is the only way to go until native boot of SSD is available.
Ok, thank you all for you great input!
I will choose the Pi 4 for HassOS/Home Assistant and at the beginning from an SD-card. When USB booting is ready in a few … (insert days, months, years .-)) I will change to it.
Thank you for the link!
If you don’t use HassOS, you have to install the Add-ons like Duckdns, nut UPS tools, flicd on your own on Raspbian?
