I looked at the Aura HD and the Paperwhite before settling on the Kobo Aura (non-HD) a couple months ago.
The deciding factor for me was weight and size. The Kobo Aura is actually smaller than the Paperwhite, both in dimensions and in weight, though they have the exact same screen. It’s a beautiful piece of hardware, and one that I use all the time just because it’s so easy to always have on me. That’s something that’s somewhat easy to overlook in devices—people want the beefiest, most powerful, speedy hardware, but if it’s too cumbersome to just stick in your bag and go, I find I use it less often. This thing is so tiny, I can stick it in my smallest purses and forget it’s there. Battery life is also a non-issue, on par with the Paperwhite.
Anyway, the screen, while not quite as high-res as the HD, is still beautiful and compares favorably to the Paperwhite. All the same font-tuning options are available as well, though I believe the HD has one or two extra faces. The capacity is the same as the HD, 4GB on board, expandable to 36GB with a 32GB microSD card.
I sideload all my books so the ecosystem doesn’t matter at all, though I do agree that the home screen could use some work. It’s possible to remove tiles you don’t want via a long-press, but it’s not immediately obvious and takes some getting used to. And contrary to the review I find it extremely handy to have a physical light-press button. Whereas on the Paperwhite you have to fidget around with a slider on the screen, on the Aura you can set your brightness setting once, and then just toggle it on and off with one keypress. Much more convenient.