2015 update: I have tried both the Kindle for PC and Nook for PC reading apps, and found the Kindle app to be far superior. As has been mentioned already, Kindle does offer a surprising variety of eBooks for $1.99 or less, including a nice selection of either collected works, or complete works, of major classical authors for $0.99. Nook does not.
While both apps allow textual highlighting, only Kindle offers the option of copying textual passages for pasting into word processing files. This works seamlessly with Word Perfect, so I would assume that it would works with Word as well. Plus, with each copied section, the Kindle app appends all of the data needed to create a bibliographic entry should the material be included in a paper or article. Nook does not offer this option.
Also, with Kindle, purchased books begin downloading immediately, and the download continues until the entire book has been downloaded. With Nook, generally that is the case as well, however two or three books that I purchased failed to download at all, despite repeated tries. Trying to get this situation resolved through Customer Service was an adventure in futility.
As to the tablets, the 4th generation Kindle FireHD tablets win hands down. More reasonably priced than the Nook, and loaded with far more capabilities than the Nook Color, FireHD tablets function like Android devices, and make virtually the entire Android app collection available, As well as third party apps, such as the Scribd.com mobile reader app.
Based on my experiences, the Kindle Fire HD tablets and the companion Kindle for PC reader app are clear winners.