The Kindle paper white can load DRM free ebooks, especially if you use Calibre which will auto-convert any format into a format that Kindle can read. Personally I think the iPad Mini has a nicer reading experience because of the larger 4:3 screen, but either should do nicely.
I use my iPad mini to do some light reading (mostly e-magazines), but for any extended reading session I vastly prefer to use my E-ink reader. Much more comfortable on the eyes. If you're talking about reading outside, I wouldn't try it on any tablet that isn't e-ink.
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