I have dilemma. Buy Android tablet? Or nettop?
I have laptop already. Things went in that direction that I need separate not-so-powerful device for video play in background or working with documents when my laptop busy.
I am thinking go with Android tablet, or small mini-pc? In case of cost those one areapproximately same or near same.
A small mini PC can be more versatile as it can function as a mini server of sorts.
What is a “nettop”?
just get a second monitor from a dumpster for free?
Thanks for the clarification.
Not offering advice, just what I would pick. The mini-pc, but replace the OS with Fedora Workstation.
I used a Samsung tablet for a a couple of years with a physical keyboard and mouse. Not a bad setup but not private. Also, not very secure with the updates that happened just a few times per year.
I need SEPARATE device that can work in background 
Of course! I always flash Linux. Hate Microsoft.
Plus recent news about impossibility to unlock bootloader and that thing with sideloading of apps block raising serious concerns.
I’d probably just go with whatever is cheapest. This is a pretty minimal workload.
I do kind of agree with @SkewedZeppelin that a 2nd monitor would be the best option. Why does it need to be a separate device?
GrapheneOS supports Pixel Tablet. The question is if it meets your needs.
Easy: You cannot run laptop 24/7. But I need PRIVATE device that will be connected to network like 24/6 (no mistake here) to receive notifications etc, etc.
And due to my threat model I try to separate hardware with extreme hardening (laptop) with normal usage at home on regular OS like Mint or Fedora.
Why? More convenient.
I know that. My question was: what better for me. A tablet? Or mini-pc?
And I won’t use vendor-one android since I 100% sure there is a backdoor (for example remote “anti-theft”, what prevents Google to use it in nefarious purposes?).
If I decide to go with tablet it will be GrapheneOS or LineageOS.
I am trying to go fully FOSS to make sure there is no backdoor (at least from vendor) and ditch piracy (comon it’s not even legal). Yes I know I can buy software, no I won’t do so by principle. I better ditch it and donate to FOSS projects.
go with a PC then, a tablet has too much proprietary apps even if you use graphine or lineage
Since FOSS is important to you, do you care about coreboot at all?
A rather expensive but power efficient mini-pc you can purchase would be System 76’s Meerkat. It comes with Intel’s IME disabled if that is important for you.
I’ll be honest, any used ThinkCentre on eBay can work just fine if your budget is tight. You can get one for around 80-150 USD.
That’s what I said.
GrapheneOS supports Pixel Tablet. The question is if it meets your needs
Do you care about security? Many mini PCs don’t receive firmware updates.
Yeah, this is better
Looked at it. Not so expensive. A little bit more than tablet. Okay, I think I found what I was looking for.
Many thanks 
Which firmware? I will flash linux. Or you are about UEFI?
UEFI firmware updates pushed by device manufacture to fix security vulnerabilities, add new features, or improve compatibility with certain hardware or software configurations.
True but its support ends in 2 years, 9 months. With the Pixel 2 tablet cancelled, its not a long term option.
Is there a simple guide or list that mentions which are better than others for firmware support? I was looking at these mini PCs as a firestick replacement and I’m currently drinking from the fire hose of knowledge. It sounds like my threat model is less intense than OP but I’m curious to know.
