Question is do you want device for reading? Or general computing like internet, youtube and email? Do you want to read magazines or pdfs on it?
Purely for reading and mainly just for fiction eReader is a good thing. If you just occasionaly want to browse net and do so on mainly text based sites you might survive with a reader which has wifi.
Tablet is a good choise if you used it for other things too like youtube, games, lot of browsing and magazines.
And yes, battery last much longer in eink devices as there is no backlight and screen is updated less often.
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