I use the dictionary a lot. That said, I find the dictionary on the Kobo and the Nook to be fine except for the grey text on the Nook. I do prefer the Kindle dictionary.
Things like the amount of space used by headers and footers don't matter much to me. I don't care about how much text is on the screen. in fact I use the widest margin on these things. Turning pages isn't an issue for me.
As for the 2 parts of the software, the reading part and the rest, the reading part matters. The rest is nice if it works well but it doesn't matter much to me one way or the other.
I read one book at a time. I rarely change fonts or sizes or weights. I set those without too much fussiness when I get a device and then pretty much forget it. What matters is the text on the screen while I'm reading and that it have a decent dictionary.
I'm actually the inventor of ereaders, way back when I was about 14, maybe 1954. One day, after a long session of reading while laying on the sofa I realized it was getting dark and I didn't feel like getting up to turn on the light so I just laid there thinking about books and the perfect way to read them. I envisioned a scroll like mechanism inside a flat plastic or wood panel with a rectangle to show the paper and a knob at the top and bottom to scroll. It was, in my mind, about the same size as my Glowlight 3. I never considered actually making one but I had the idea first. Of course it wouldn't be electronic, since integrated circuits were still years in the future, to say nothing of e-ink. Actually it would have probably been a foot thick but in my mind it was half an inch thick or less. I had a pretty good imagination.
Barry