With paper books I'm more concerned with having to have space to put it when I'm done. But, I find it more enjoyable to read a physical book, and am more likely to re-read it.
I'd love to keep them all forever but (a) I think my house is about maxed out on shelves and (b) a book's potential is wasted on the shelf.
My current strategy is this. If its a book I know that will have re-read value that I really enjoy. I get a physical copy (such as LOTR). If its an interesting book that I want to read, but I don't see it as having high re-read value, I buy it on my kindle. May not be logical, but its a system that works for me so far.
If you later find out you want the paper book as home ornament, then spend a little extra to decorate your home.
The two major mistakes we make about books is:
1) we think we have to finish it. No, drop it if it’s boring.
2) we think we have to keep it around. No. Take a picture and write a few notes for your personal blog if you need to keep track. You don’t need it in your room or house.