You may find this
matrix of ereaders helpful.
Which device you should get really depends on what you're looking for. Is there anything in particular you want the device to do?
I have a Kindle and love it, but I'm pretty sure I'd love just about any ereader. The things I especially love about the Kindle are: the dictionary (which I never thought I'd use), ease and quickness of menu navigation, highlighting passages, and the look/feel of the device.
However, the Kindle comes with the caveat that it doesn't read ePub files, which is a major ebook format (and possibly one day the ebook standard format?).