Pros: Wilton store bought fondant is convenient---as in, it saves time and energy. Easy to work with when making flowers and decorations. Also easy to color with the gel pastes.
Cons: Expensive! $21 at Joannes and $18 at Walmart. Tears easily. Tastes awful.
Pros: It stretches easier than Wilton fondant, so it doesn't tear as much. Better-tasting. Cheap to make ($5 compared to $20!). Super easy to color only if you're making one batch of color at you add the coloring at the right time.
Cons: You have to make it yourself--very time consuming. And its kind of a pain to make.
As you can see, marshmallow fondant is the way to go. IF you have the time. Which I sure don't. So I compromise. I use marshmallow fondant for one-tiered cakes 9" or smaller, and the Wilton fondant for cakes with more than one tier. There's always 40% off coupons!