What is the difference? or is there a difference?
Marshmallow fondant is just one recipe for home made fondant. The recipes are pretty similar, powdered sugar, gelatin, moisture. (marshmallows usually have gelatin)
Ready made fondant is easier for beginners, or if you are in a hurry. The exception is Wilton. It handles great but tastes bad. If you want to buy fondant, there are many good brands.
If you decide to make MMF, be sure to hold back some of the powdered sugar. It can easily get too dry,