Depends on the type of book. Novels, I like digital (or audio), cookbooks and other reference texts, I like physical, even though it's usually easier to ctrl+f to find stuff.

If it's a book I know I want to actually own, I'll buy a physical copy. Basically, my criteria there is if I expect it to be a reference I'd use multiple times, or if I expect to lend it to friends, or if it's an author I want to support.

My preference is library for most books. I'll scan thrift stores for books to buy, though obviously no guarantee of any books there. Local used bookstores are next, then local new bookstores, then online used book stores like thriftbooks.com or abebooks (though that's an Amazon subsidiary). I rarely get new books online.