well
If you want it mainly to read PDFs books I think DX is the best for that because
it has a big screen so it will show the PDF books correctly. - e-readers with 6" screen will destroy the file it will look bad especially if it contain formulas or photos like math physics -
If you want it to read book from B&N or amazon store both K2 or Nook will be just fine
I didn't try the K2 but I do have a nook and I'm very very happy with
I read the comparison between K2 & nook which you can find it here
http://www.barnesandnoble.com/nook/c...?cds2Pid=30195
then I decided and bought a nook.
The other thing let me take nook instead of K2 is that I find Nook very very beautiful
and designed well. K2 its like an device from 1990s
my final advice ''take a nook you will just love it like others''
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good luck and sorry for my bad English