If it's a new book and one that I think other people may be interested in borrowing, I'll get the hardcover for the extra protection.
However, there's a used paperback store down the street from me that has a whole bunch of heavily used paperbacks for like $1 each, and those have definitely been dominating my collection lately. Sometimes I'll just pick up a dozen of them. That little store is one of those treasure troves of unexpected things, even though when you find one of those treasures, you might need to flip the pages carefully to prevent it from falling apart.