If it's a book I just can't find in physical form then I will go the ebook route. This is definitely the case for those out of print books.Sometimes I MIGHT be lucky and find it at a second hand bookstore. If it's a new book I really want I will buy a hard copy and read it like that. I still am a collector of things so a physical copy is nice. If it's a book i'm not overly excited about then I would go for the ebook version, yet I still find hardcopies more convenient than carrying around something I might need to charge. I wouldn't buy both though.
Let me know in advance. I can send you my spreadsheet which lists all the books I have and don't have.