Which phone to choose? Galaxy S8 or Xiaomi Mi Mix 2 or Xiaomi Mi 6?

1. New / used.
2. What brand / model do I prefer or have used so far.
3. Construction: standard, flap, sliding.
4. Operating system: symbian, android, windows, other.
5. The keyboard should be: standard, qwerty, tactile.
6. Display size.
7. For what you mainly need a telephone.
8. What functions do I care most about.
9. What functions are completely redundant or less important to me.
10. Conditions / places where the telephone will be mainly used.
11. Estimated budget.
12. Other requirements.

AD1. New,
AD2. I currently have an Asus Zenfone 2 Ze551ML,
AD3. Standard,
AD4. Android,
AD5. Touch,
AD6. 5 "and more, I don't mind,
AD7. Internet, social media, snapchat, instagram etc.
AD8. --------
AD9. ---------
AD10.
AD11. About PLN 2400
AD12. Lack

I was thinking about Samsung Galaxy S8, Xiaomi Mi Mix 2 or Xiaomi Mi 6
I can't decide which phone to choose. On the one hand, I like Mi Mix 2, but the Galaxy will have a better camera. Maybe some other? What would you advise?

https://www.youtube.com/watch?v=p7rfvzRz6lo

Go ahead and take the S8 because it is a very successful and cool model whose advantage is a battery that can last a full two days with fairly intensive use. The camera is in a class of its own and is also quite resistant to falls.

How about the S8 with this pink screen problem? Does it happen often?

Can phones from European distribution, which can be bought on a well-known auction site, be bought without fear? They are much cheaper on the other. For example (auction number 7009826077)

If you try to see this rose by force, you will even notice it on a purple sweater. I watched my mother-in-law S8 and this effect does not occur. However, for example in A5 2016, I see pink on a white background, when I am under the lighting of fluorescent lamps. In my opinion, you have nothing to worry about.

Added after 1 [minutes]:

As for the "occasion", my friend @ tino2003 devoted a separate topic in the section to it.

I have read about these "occasions". It will be better to buy used with warranty.

What to look for when buying a used Samsung S8 / S8 +? Apart from possible damage, scratches to the device itself

What it looks like with redeemed phones with network branding without simlock. Can the network somehow block it?

Do not buy with tmobile and plus because these networks block phones for not paying the subscription. Search with play or orange with an invoice of 23% because this is the only one that Samsung honors with a warranty.

I still have a question. I found an auction with phones from "Polish distribution" no. the auction "7090695737".
Thanks for your help so far.

I also found such "7112392055" I need a copy of the original invoice with orange for warranty?

The first is from European distribution, there is no mention of invoice, there will be only one warranty repair at best. The second auction is ok, you have the warranty card with invoice (documents style for orange). Even if these are phones from Polish distribution, more and more often the services check the warranty status after imei, without using the documentation. But you can go for this European distribution in this case.

I found out that it adds an original Orange invoice to each phone. The phone is from the Polish Orange store.
Auction okay?

Phone bought. I got the original invoice and the manufacturer's warranty. The box was closed with an original seal. Thanks for the help. Best regards.