Enumerator Implementation: Use struct or class?

I noticed that List<T> defines its enumerator as a struct, while ArrayList defines its enumerator as a class. What's the difference? If I am to write an enumerator for my class, which one would be preferable?

EDIT: My requirements cannot be fulfilled using yield, so I'm implementing an enumerator of my own. That said, I wonder whether it would be better to follow the lines of List<T> and implement it as a struct.