I don't know about the kindle, but I have a nook so I'll tell you about that.
When you buy a book it is always on there unless you manually delete it. There is an option (don't know if kindle has this) where you share books with other people who have nook's and after like 14 days it either locks or gets deleted. (not exactly sure how this works...I've never tried it)
I got an e-reader for the same reasons as you! Too many books, but don't want to get rid of them. I can't really tell you which one you should get. I think it's all in personal preference.
One other thing, if you buy a kindle, you can't read any nookbooks on it. And vice versa. With my nook, I can't read kindlebooks. Just something to consider. You can also check and see if your local library has ebooks. Mine does and you can rent them.
Sorry for the rambling...hope this helps.
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