With the original Paperwhite, Amazon flirted with 2 GB internal memory, but they quickly backtracked and every other model (including all current models) has the usual 4 GB.
A Kindle has about 3.25-3.50 GB available for sideloading books -- that means you can store on average roughly 3500 books on a Kindle.
I personally cannot fathom why anyone would need expandable storage, unless they read a lot of manga or other unusually large ebooks.
(PDF tends to the larger side as well, although that format is to be avoided if possible...)
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Stay away from the Nook. Barnes and Noble has been doing everything they can to act like they are going out of business. e.g. they finally outsourced their development and tech support to India, their website is a constant disaster, they are breaking the vaunted ADE compatibility
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