@Lami
I answer here: i brought the LG SJ810v (49)... It was already preodered before you've said that the Samsung was better even if it's 60hz

The main issue was the 21 vs 31ms input lag, but i don't really feel an input lag that consequent with my settings
At first, i'll enabled the HDR RGB for the PS4 pro recognise it.
On HDR, i precognise "standard" for the picture settings, desactivate "True Motion" (i think it may cause input lag) and all things that could cause issues with the ms and framerate (like MPG thing etc. in Advanced settings)
There's one thing that bottering me: "Game mode": i know it's calibrated for having the less input lag possible. But the issue is that the LG "Game mode" version is a bit too dark in terms of constrast. LG is probably planning a firmware update as i've readed about, to correct it.
So if you want to play with game mode, you need to enable the "Dynamic contrast" to the maximum... I don't like that much dynamic contrast and desactivate it on every TV i've got, but no choice for the moment.
And there's also "Standard" mode (Dynamic, Cinema, etc. you know), no issue with the picture contrast with exactly the same settings. Because the rendering is different > differents settings for colors, luminosity, temperature etc. but the rest is the same, everything that could cause lag is desactivated.
But i don't know if it's my imagination, but there's sometimes a bit of lag in the image (not really "input" lag), some slowdown on PES when there's an "Highlight" or whatever...
Only solution: to activate "Lg True Motion", which make the game looks way better in movement... But i don't know how much input lag it may cause.
So, 2 options
Option 1 / Game mode
+Probably the better setting for input lag
-Obligation to enable "Dynamic constrast" otherwhise the image is very tern, like there's White vs Black isn't that much pronounced.
Option 2 / Standard Mode
+Better picture, no doubt
-Some very little "stuttering", but no stuttering if i enable True Motion
? I don't know if True Motion will gives me more input lag (probably yes) but the flow of the image is outstanding, more FPS, better transistion between picture to picture.
And i don't really feel a big difference in input lag to be honest...
So, i switch from "Game" to "Standard" mode. I'm very happy with the TV you've recommended, without doubt it's a game changer for me at this price

I'm only thinking about "what setting is the best" for gaming experience in you opinion?
I really hope they will update soon the Game Mode: even if i don't really feel the input lag, it's more "to be sure of not having issue with it.
Perhaps i will feel it on SFV (got the game but didn't test), but on Dark Souls, PES, Fifa, Nioh... It's barely noticeable, i don't notice the big difference between the 2 options i switch between.