And it depends on how you want to store your books.
The Kindle has 4 GB of memory which holds on average 3,500 books.
I don't know the storage limits of the Sony or Nook but they both have memory card slots so you can expand the storage space.
One of the things I like about my Kindle, and I think the other major bookstores that sell e-readers do this as well, is that I can read a book and remove it from my device. If I want to read it again I can reload it to my Kindle from Amazon for no charge.
Right now I have plenty of space left on my DXG, I have 400 or so books, so I don't worry about deleting anything from my DXG.
I would be more worried about how will I be able to organize my books on my device and where do I want to buy my books.