books take longer to read on the smaller readers because the books have so many more pages on them.
that can be a good thing or a bad thing depending on your point of view.
1. is larger than 2. is larger than 3. (7.8" -> 6.8" -> 6")
Waterproof is best to ignore, electronics like to keep dry either way.
It's too early to judge the Kobo Aura One... give it 2-3 weeks for people (not just reviewers) to actually get their hands on them and tell us their experiences.