I prefer gum paste--my personal preference is Wilton's pre-made gum paste-- when making things that need to dry quickly and hard. Bows, flowers, any kind of topper that needs to stand etc.
You can use fondant for all of those things, but fondant takes longer to dry and to me, you can't get fondant really thin without it breaking...that's what makes certain flowers look so delicate...the thin petal like qualities you can't get with fondant. Also, gum paste items can be a lot lighter than fondant items which is nice for things that need to be applied to the sides of cake...like cascading flowers.
The downside to gum paste is that you have to work fairly quickly with it or it will start to dry and crinkle when you shape it. Always keep it covered.
Some people mix tylose with fondant to get it to stiffen and harden faster, but I don't care for it. The fondant doesn't seem as pliable and items are heavier. But this is personal preference.
I sometimes mix gum paste with fondant if I want something to stay pliable a bit longer and when I have the time to dry things.
I use fondant for decor when I know it will be eaten, like polka dots, stripes, etc. Gum paste is edible but not so tasty!