I have a Kindle PW2 and an Aura HD. I don't currently use collections on either device, so can't comment on that. I really like both devices, but if I had to choose just one, it'd be the Aura HD. I like being able to customize the fonts/margins/line spacing for what I'm reading. Honestly I don't notice much of a difference in size unless I'm picking them both up at once, although the PW does fit in the side pocket of my purse easier. :-)
The one advantage the Kindle has over the Kobo is the dictionary. I find the Kobo's adequate and it's very rare that it doesn't find the word I'm looking up. But the Kindle has far more in depth definitions.
Now, I love my HD, but I received it as a birthday present just before the H2O came out. If I were purchasing today, I'd definitely go for the H2O.
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