These days I tend to get physical books when:
* They're significantly better "things". I have some lovely books that are lovely objects in of themselves. Well typeset, bound well, quality paper, folios, etc. For example my hardcover of "The Timeless Way Of Building" is a beautiful object — as well as a wonderful book. I like having beautiful objects around me.
* When the format doesn't work as an e-book. For example Stephen Cole's "The Geometry of Type" would be completely useless to me as an ebook. As would most of my books on photography art and architecture. They're not just lumps of text that can be trivially reflowed. This is something that might change if we get some really high-quality A3 portable ebook reader type things in the future.
* When I can't get them as an ebook. There is still a stupid about of immensely useful content out there that you cannot get in ebook format. For example I'm currently finishing up a re-read of Victor Papanek's "Design for the Real World". Which you cannot get as an ebook.
That said — my default for fiction/reference purchases tends to be ebooks now. With occasional upgrades to paper books if they're something I really liked and I can get a "nice" physical edition.