If you look at a side-by-side, they're pretty evident (I compared my outgoing 6P to my 5T).
I don't really notice it during daily use though, having moved from a long line of 4k display phones (my last 4 devices) now, I can say my 5T display is good, maybe not awesome, but solidly good.
Sure, you can't really see the pixels unless you're ultra-close, but the smoother lines and less dithering of the 4k display is evident, if you ask me.
I think the color temperature (even though I have mine on one of the "warmer" settings on my 5T) of the display is quite a bit cooler (higher value) too. I really seek out warm LEDs(2700-3000 max), in general, so this may be something not everyone notices.