If you live in the US, go to Best Buy and compare the Sony, Kindle, and Nook. That will give you a good feel for each of the e-readers and should help a fair amount.
Collections on the Kindle is a tagging system. You build your own collections and put books into the collections. The big problem with Collections is that it is only one level. So I have To Be Read Fantasy and Read Fantasy. I would love to have sub collections in that folder for different series and authors but that is not possible.
I have no idea how the Nook book shelves work. I believe that Sony does allow subfolders.