To start, here is a set of photos that show the Aura HD next to the Aura H2O. As you can see, there's an obvious difference between the screens; the Aura H2O has a much whiter screen.
There are 4 images, with the last one being a composite image made from a sample of white from each of the two screens.
Both ereaders have the same touchscreen tech, but not quite the same frontlight tech. I've been comparing them, and I can tell you that they are equally responsive and turn the page at the same speed (which is slightly slower than the Kindle PW2).
Given that they have identical CPU specs, and that I had updated both firmwares to v3.8, this makes sense. But when it comes to the frontlight, I can see an obvious difference.
Both devices have a 1% setting, but the Aura H2O is much dimmer at that setting (yay!). This means it is throwing less light in your face, which is IMO a good thing.
Here are 6 shots, from full brightness down to 1%. In case you were wondering, the Aura H2O looks fuzzy in the last shot due to the dimmer frontlight. I can confirm first hand that it was still very readable. The Aura HD is on the left.
And it's not just the screens that are different; there are also some obvious and not obvious differences. For example, the Aura H2O doesn't like any of the USB cables other than its own: the Aura HD is less picky and will work with any cable that has the correct-sized plug.
The Aura H2O is also thinner, has a rubberized rear shell, and is waterproof. It meets the IP67 standard for dust and water proofing.
I'm still working on the review, and while I am not ready to give a final opinion I can make a couple recommendation about the Aura H2O vs its immediate competitors.
With a retail of $180, this is a premium ereader and it really only has a couple direct competitors that are readily available. One is the Kobo Aura HD, and the other is the Onyx Boox T68.
First, if you're trying to decide between the Aura HD and the Aura H2O, I would get the latter model. It is absolutely the better device in many ways, even if you don't care about the waterproofing. It's worth the extra $10 on the retail price (but if you can get the Aura HD as a cheap refurb, that is a different case ).
As for the Onyx Boox T68, I can't offer a definitive answer for which one you should get. They have the same screen size, but the T68 has the older Pearl E-ink screen tech. I liked the open Android on the T68 when I reviewed it in June, including the way it lets me install many different reading apps, but the Aura H2O has a nicer design and better frontlight controls.
If you want to do more with your premium ereader than just read ebooks, get the T68. But if you want the best Epub ebook reader, get the Aura H2O.
It is up for pre-order for $180, and is scheduled to ship at the end of the month.