I think you need to refocus on what you need and not get caught up in minutiae that will turn out to be unimportant in the final analysis.
1. Go to the Kindle store, and to B&N again, and to the Kobo store, and make sure that you can buy the books you want. Amazon has more newer books than all of these stores put together...but not by a lot, and it's quite possible that you won't be able to get the books you want in e-book format anywhere. This is the most important step.
2. Remember the DRM's EPub is no less restricted than Amazon's format. It doesn't matter that Epub is open if you are buying DRM'd epub.
3. The Kindle store has 75-80% of the US market. Sony has about 5%, and B&N has the rest, with a few percentage points left over for all other publishers.
4. Look at the readers again. K3 screen is a Pearl e-ink screen and notably better than the Nook's older screen.
5. Think about getting an iPad (or Samsung tab) with the Kindle App and the Nook App.
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