What is the difference between fondant and gum paste? Which would be best used for a large bow to top a wedding cake?

You'll be able to roll gumpast thinner and it dries super fast. A pasta machine is great for rolling things super thin.

fondant is used for covering cakes. it can also be used for accents and modelling. takes a long time to dry out, so for things like bows, its not so great, and it doesnt dry as strong as gumpaste.

gumpaste, although edible, isnt exactly tasty for most people, and dries so hard, you wouldnt want to cover a cake with it. it is good for modelling (some like to mix it 50/50 with fondant) and excellent for things like bows. it gives the most realistic flowers as well, as you can roll it so thin.
hope that helps a bit.
xx

for bows I mix gumpaste and fondant together. about a 40/60 mix works for me.