If you are willing to liberate your books from their DRM (it's very easy), then you don't need to consider the sharing schemes. If not, you're better off with a Nook or Kindle.
The new screens on the Sony and Kindle are both equally nice, IMHO (
clicky), although I much prefer the way epubs display.
Also, if you're willing to liberate your books, you can get a Kindle and not be limited to Amazon. You just use calibre to convert epubs to mobi (just hit a button). Mobi files converted using calibre are perfect (you wouldn't know that you didn't purchase them in Amazon's stupid format). Same goes if you decide to go with something other than the Kindle and would like to take advantage of the free Bibles on Amazon (you just get them with Kindle for PC, liberate them and convert them).