These days reading are primarily of 2 types,
1 online ( articles, magazines, news, blogs, reviews etc )-fast changing, short sittings, fresh/dynamic content, interactivity and multitasking.
2 offline ( books, word documents, pdf ) -slow and static, long sittings , refined content, no interactivity and just reading.
IPAD mini is designed for the 1st type.
KINDLE is designed for the 2nd type.
now you can decide what you want more and invest accordingly.
here is a video comparison of both the devices.