Other factors I'd add
- how are your hands? I have very arthritic hands and the Clara is lighter and smaller so easier to hold, though the back is somewhat slippery (but probably improvable with a skin). The difference is not huge though, and even on the go I've read on the Clara one-handed while gripping a bus pole - for short trips.
- how do you prefer reading? If kepubs, with margins, header and footer, then the One is better. If you don't mind epubs with no margins, the real estate might be less important to you.
- how are your eyes? If you can read comfortably with a smaller pitch or narrow font, then Clara is fine.
- what do you read? If you read books with long sentences, poetry for instance, or German where you have to see the verb at the end to make sense of what you read, or books with charts, tables and such, then the One is WAY better.
- how long do you read? I'd say that for half an hour or an hour a day, Clara is mostly fine. For longer reading sessions or complex books, I much prefer the One.
The look of the page is important to me, I read books of all kinds and I have aging eyes, so if I had to choose just one reader, I'd choose the One.