The main difference between Italian Meringue and Marshmallow Fluff (or marshmallow creme) is that Marshmallow Fluff contains corn syrup for even more stability. Marshmallow fluff has a stiffer texture and is stickier and more difficult to spread. In my experience, it may also be more likely to contain more vanilla to replicate the "marshmallow" flavor. Use Italian Meringue for frosting cakes and cupcakes, and topping pies and squares. Use Marshmallow Fluff for swirling into brownies, or using as a spread to make fluffernutter sandwiches or s'mores and s'mores-inspired desserts.