I would say that comparing these two is like comparing apples and oranges. They are designed to address different problems. Rails’ persistence engine is made with the aim of being as easy as possible to use. Hibernate feels more like it’s trying to be the kitchen sink of ORM tools. That doesn’t have to mean it’s bad in any way, just that it’s trying to address and solve a bigger and different problem than Rail’s built-in ORM support.
So in a way it’s quite unfair to compare the two but it does highlight the differences.