I have got my OLED Steamdeck today and made a picture with my old one side by side. The display is a little bit bigger, the touch screen is way better (more like a smartphone).
I didn’t try this out on my OLED steam Deck yet, but dark scenes are a bit of a strain on the eye on the LCD one. I played mark of the ninja on it and the black and darker stuff (which the game has a lot of) is illuminated like any other color, which feels a bit unpleasant. With OLED black is the absence of light, which feels much more natural.