I have a Kobo Glo (not HD) and an Aura (also not HD) as well as a Paperwhite 2 and a Voyage. Frankly they're all so good I'd be happy with any of them as my only device.
However, comparing them, yes the Kobo's allow far more adjustments to fonts and margins, etc. than do the Kindles, but the Kindle screen and text still looks better. No adjustments on the Kobo's can make the text as readable as either of the Kindles.
That isn't to say the Kobo's are bad. They're not. They look really good. The Kindles just look better.
I haven't seen the Glo HD or the Aura HD so I can't really compare them. I'm just giving my impression of the ones I have.
I read with all of them and I find that I really don't much care which I'm using. I prefer the navigation features of the Kobo, but not by much. I prefer the screen of the Kindles, but not by much. And when I'm reading I don't notice any of that. As Jeff Bezos says, these things disappear.
By the way, I like the Voyage more than the Paperwhite because it's smaller and lighter and feels better in my hand. But the higher resolution screen really doesn't have any effect on me. When I put them side by side and compare the Paperwhite looks just a bit better (warmer color) to me than the Voyage. When they're not side by side I can't tell the difference.
Barry