I’ve been using Sony’s oldest Reader model, the PRS-500, for a little over 2 years now and I still love it. It gets the best battery life out of all the readers (despite it’s age, I still get nearly 3 weeks of use on a single charge).
When making a decision for which eBook reader you want to buy, the most important consideration is where your books will be coming from. If you’ll mostly be getting books from the public domain and ahem less-reputable sources ahem, then save the extra cash and get a reader without wireless purchasing capability. However, if you like to read the newest releases and hate hooking things up to your computer, there’s no beating the convenience of browsing and purchasing directly on your device.
If I was choosing between the Kindle and the Nook, I’d go with the latter. The refresh rate on e-ink screens is not great. It’s wonderful for book reading, but not great for internet browsing. The Kindle’s book browsing happens on the e-ink screen, which I imagine might be clunky. On the Nook, however, the book browsing is done on the color screen, with normal refresh rates and an experience similar to desktop internet browsing (or more accurately, smartphone internet browsing). All internet and device navigation happens on the color screen, which would definitely make the entire experience smoother. Plus, you can type right on the color screen for notes/annotations, dictionary lookup, search, etc. In my opinion, it’s better than having a physical keyboard.
In general, when purchasing any electronics, if you have the option to get a device that has expandable memory and replaceable batteries, always go for it. You never know if/when you’ll need more storage space and you don’t want a faulty battery to force you to use a company’s costly repair service when you can buy a new one and change it yourself.