I'd choose the latest PW2 over any other reader due to the quality of the light/screen and the general speed & responsiveness of it compared to others (not huge differences to be fair!), added to how stable they seem to be - though to be fair, neither my Kindle nor my Sony have ever frozen up on me more than once each. In terms of battery, I got 3 weeks out of the first charge on my PW, and I think the Kobo's would probably last a similar amount of time.
I took a long time considering the Aura, but while some people have bought it and are happy, looking at video comparisons of it and the PW2, I know I wouldn't be - but again, they are not huge differences, I may just be a bit picky!
However, I didn't consider the Aura HD because I'm not interested in the bigger screen. If I had been, the Aura HD would very much be an option. While you still hear of issues with it in terms of firmware bugs, freezes etc (the Kindle PW is far more stable but it isn't perfect either with the one firmware update released annoying people with collection issues), the screen & light looks better than on the Glo & Aura, it just seems a step above anything else Kobo have on offer.
If you were happy enough with your Kobo, the most obvious upgrade would be to try the Aura HD, but in all honesty, if you mainly buy your books directly from one shop and load them onto your ereader a few books at a time as you buy them, then just read and don't fiddle too much with settings, I'd guess you will have little issue with any of the Kobo's or indeed the latest Paperwhite.
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