I'm a newbie who just used MMF for the first time. Although it does taste better than ready-made fondant, I'm wondering about using a crusting buttercream next time. What are the differences? Will I still be able to "roll it out" or do you have to frost the cake like regular BC? Thanks!

Buttercream, crusting or not, is totally different from fondant. No you do not roll out b'cream.
Crusting b'c will 'dry' (some say 'set up') to where you can touch it w/a finger & it will not come off on your finger. Non crusting never dries or sets up.