What is the difference between gum paste and fondant? Is one more pliable? Does one taste better? Which works best for cookies?

Fondant is usually used as an icing for cakes. Yes, it can be molded, but it isn't always very sturdy. You have to make sure you make a piece in advance if you want to mold and/or shape it so it has plenty of time to dry. You might also want to add some gumpaste or tylose powder to it to help strengthen it and make it more pliable.

As for gumpaste, this is generally used for molding pieces, flowers, etc. It's not used to cover cakes. It dries to what I feel is like a porcelain consistency and tastes like those old time Nekko candies (not very good). It is edible, but since the taste is not favored by most, some people say it's inedible. To me it's just not worth eating due to it's taste and texture and the items made from it can be saved (with or without spraying them with a laquer).

I hope that answers your question.