How portable do you want the device to be. The iPod Touch fits in a pocket. If you ALWAYS carry a purse, backpack or always where clothing with large pockets, then a iPad mini is able to go with you everywhere.
If you already have a Smart Phone, then a lot of the portability you get from an iPod Touch is available from the smart phone.
Reading ebooks can be done on an iPod Touch (I've ready hundreds of them on mine). However, if you need to use the device for reading text books or PDF documents with diagrams, then the larger screen of an iPad mini is better.
Think about what you want to do with the device. If you can get to an Apple Store or another store that carries both products, play with them while thinking about how you would really use them.
I own an iPod Touch because to me, having it in my pocket and with me all the time is more important then the larger screen. And since I own a Macbook Pro laptop, I have a larger screen for PDFs, movies if I want a larger screen while away from my home TV and Netflix, I do not need the iPad or iPad mini larger screen.
But an iPad is ideal for my Mother-in-Law. It all depends on how you want to use the device. And only you can answer that.