What is the difference between fondant and rolled butter cream?
This is not the most accurate answer in the world but for me:
Fondant is based on sugar/glucose, is pliable/kneadable and you can roll it out or mold shapes and figures with it.
Rolled butter cream is based on buttercream that has more sugar added to make it firmer? - I have never used it, so can't tell you how to use it.
I guess you use the medium that would be most suited to the job you've got. Hopefully someone else will be able to answer your question better...
i do know that rolled buttercream is supposed to be more fragile to work with. . . haven't tried it myself. good luck