The only reason I'd ever get a Kindle DX is if I read a lot of pdfs or magazines (NOT books) and wanted a true e-reader (with e ink, not a backlit screen). The market for DX users has ALWAYS been small, and I don't imagine it will ever grow. In fact, I am not sure how much longer they'll even make the DX. It's sort of an inconvenient size, IMO.
The Kindle Fire is a tablet (no camera, no expandable storage, but still a tablet) that has a reading app on it. It is NOT an e-reader. It does have a very competent web browser (I prefer it to either of my laptops, in fact) and can play games and other apps, show videos, play music, etc. I WILL read on my Fire in a pinc, but 99% of the time I have one of my actual Kindles with me and I read on that (or, in the case of the dark car ride...read on my phone).
My advice would be this:
if she wants a portable computer/tablet that has the capability to read books on it occasionally, get the Fire.
if she wants an e-reader, get a Kindle (one of the e-readers with 3G, either the K4 Touch or the K3 Keyboard). (Although I wouldn't use them as a browser for normal, every day use. Again--just like I don't read much on my fire, I don't use the internet on my Kindle)
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