Personally, I think the Sony Reader PRS-900 blows away the nook big time.
I think the PRS-900 should be looked at by anyone who is serious about buying an ebook. It may suit your purpose better than the nook does. I found that the eink contrast was not as good as the nook although it does have a larger screen. The case design is not as ergonomical as the nook just functional and it feels heavy and clunky. The nook is not and the design is easy to hold for lobg periods. They advertise the Sony as supporting Word documents which is only a half-truth. It does and will read Word documents but they have to be converted first. Well, any ebook reader will read word documents if they are converted to the proper file format for the reader you are using. What I do for the nook is take the Word document and make a PDF and it reads just fine. The other negative about the Sony is the huge $400 price tag. I personally think $260 was a little more than I wanted to spend on an ereader and so I could not justify the extra $140 for the Sony.
There are LOTs of ereaders out there and this weeks CES show reveals even more are coming. I didn't read about anything coming that isn't a clone of either the Sony or the Kindle. So, as far as I'm concerned I think the best choices in ereaders come down to the Kindle, Sony or the Nook. I chose the nook for several reasons. Some of which I listed above. I know a lot of people who love their Kindles and I know a lot of people who love their Nooks but I have yet to meet one person who actually loves their Sony. I wonder why that is since they were the ones to kind of pave the whole way? Kindle took the ereader to a new level, and I think the nook potentially can be, and probably will be, better than the Kindle once they perfect the operating system's software.