STI or MTI design ?

I am trying to implement a directory structure (Libraries can contain other Libraries or Documents) using the plugin 'awesome_nested_set'

if lfe was simple (only Libraries) it would be over already, but I don't see yet how to design the models to handle also Documents in the tree ..

should I define an Item model, with acts_as_nested_set and have Library model and Document model inherate from Item ?

or work only with the Library acts_as_nested_set and manage the Document as a List ?

some guidance would be welcome ... ( a lot..)