I have both a Kobo Aura HD and a Kobo Aura. They're both lovely, but I find myself reaching for the Aura HD much more frequently. The bigger screen is nice.
And they do both have Pocket (in fact, they have nearly identical features, near as I can tell, and they run the same firmware version).
Big plus for the Aura is the ability to swipe up and down with two fingers to adjust brightness, and a need to refresh the screen much less frequently (default is once every CHAPTER). The Aura HD requires you to go into a menu to change brightness, and I've set my screen refresh to every page, although you can get away with every few pages before ghosting becomes an issue.
On the other hand, the night-mode hack (which allows inverse video for reading at night) works flawlessly on the Aura HD; it's buggy (with lots of non-inverse-video screen artiifacts if you try to mark text, etc.) on the Aura:
https://www.mobileread.com/forums/sho...d.php?t=212162
In terms of cases, by the way, the official Kobo Sleepcover for the Aura HD is really nice; the official Kobo Sleepcover for the Aura is uncomfortable, and I don't like it at all -- I've now removed it, and am running without a case. (The big plus of the Aura is the totally flat bezel -- an effect ruined by having two bulky, ugly, sharp-edged center-side clamps holding the Aura into the Sleepcover; it's a design used on no other Sleepcover, and it's very uncomfortable to hold.)