Basically what Braen posted. In terms of tablet vs. physical book, the tablet wins out for some people because of the convenience factor - the weight argument against using a tablet does not necessarily apply to the iPad mini or other 7"ish devices, in my opinion.I don't understand the reading on tablet thing. My iPad is great and all, but if I'm sitting down to read a book I'd rather it look as close to paper as possible. eInk devices are super fucking cheap, if you enjoy reading for pleasure you should have one.
As far as getting an e-Ink device over the mentioned tablets goes, I see your point, but due to personal preference and wanting to use it for activities other than reading, the iPad mini hits several birds with one stone, so to speak.