Some of it is what you get used to.
I could read just fine on my PRS-600 an preferred it to a tablet.
I bought my mother a kindle 3 when they came out and found the screen to be much superior, but after going back to the PRS 600 I quickly became accustomed to it again and preferred it to the kindle because of ease of use and collection convenience etc.
Now I have a T3 and an Aura HD. The Aura HD may have a slight edge screen-wise, but the inconsistent page turning (skipping, backwards instead of forwards etc.) , flashing page refreshes, abysmal speed in opening collections and other wee quirks are an irritation I don't need when reading for relaxation.
The only complaint I have about the T3 is that it doesn't have a global text options like the Kobos. That option would make it pretty close to perfect for me.
I don't like reading with tablets, but I guess I could if I had to, just as I occasionally read with the Aura HD when my mother visits and leaves her kindle behind. (She is 88 and tells me the T3 is "A dear little reader" and pets it when she closes the cover so I know what to get her for her upcoming 89th birthday
)
Overall it is pretty subjective which screen is better. I get used to a change going from a 24 inch monitor to a 15 inch laptop screen in a day or so , and with an ereader the transition time is usually measured in minutes.
A good screen is important, but IMO the best screen is not necessarily the best reader.
Helen