Factories recreate the whole universe for every test, making them slow in large applications with many associations, and cross-references have to be worked around. Fixtures are fast, support cross-references natively, and cover the common case where a plain user or plain object is enough. Bonus: fixtures can be loaded into the development database, so exploratory-testing scenarios can be captured by adding a new fixture and reloading. A gem exists that lets developers use factories like fixtures for those who want both.