Why wouldn't they be?
He doesn't have or want an iPhone, and you can't inconspiculousy pocket a Mini year-round. So the smaller sized device is way easier to have with you always.
On the subject of iBooks, I have always read them on my iPad2, but recently I did so on my iPhone 5. Despite having to turn page more frequently, and its just a tap on the side of the screen, I didn't find it any less enjoyable on my iPhone.
Its hard to say which is best, but bear in mind that the bottom Touch comes with 32GB while the bottom Mini only comes with 16GB.